The most common problems related with chanter reeds are bad sound and no tuning. In the case of drone reeds, stopping is the most common problem. I have also a (small) list of reed makers for Galician bagpipes.
The bridle (xugo), made of metal, cannot be moved up or down in a Galician reed. It can, however, be squeezed to slightly open or close the reed lips, thus changing the pressure necessary to make it sound, the tone, and the tuning of the reed. All changes to a reed should be made very carefully, as some of them can completely spoil your reed.
Regarding sound, a good reed, when blow outside the chanter, should sound similar to a crow or to a pig -- nothing really charming. If you blow harder, a steadier sound should come out. This reed will very probably have a nice, powerful sound. Reeds which sound too easily and give a steady tone right from the beginning will have a poor sound when plugged in the chanter.
If the reed is too weak (and the sound is weak, and it tends to suddenly jump octaves while playing the two bottom notes), opening the lips by very carefully pinching the blades, near to the bridle, with your fingers and/or squeezing the bridle (preferably with a tool). Conversely, if the reed is to hard for normal playing (remember, if you are already a GHB player, that a gaita should be softer to blow than your instrument), the reed lips can be opened by pinching the blades / squeezing the bridle sideways. These actions will affect chanter tuning as well: opening the reed will eventually lower the tuning of the chanter, specially the high (top hand) notes, and will make second octave difficult, if not impossible. Closing the lips will produce the opposite effect.
Chanter tuning can be controlled as well by inserting the reed more or less deep into the chanter. In any case, the reed must always be firmly attached to the chanter. This can be accomplished by wrapping the bottom half of the reed with a convenient amount of thread. Inserting the reed deeper into the chanter will raise the chanter's tuning (mainly the top notes). Tuning a Galician chanter is a delicate task (impossible, if the chanter is of inferior quality): consider that there are as many as four standard fingerings for the high C, each having its own sound qualities. Ideally, all four should be in tune.
Scrapping the reed should be made with the greatest care and as last alternative, as this is a non-reversible process. Scrapping weakens the blades, and thus make them less rigid. The result is that the reed vibrates with less frequency, and the tone is lowered (this is not exactly true, as the exact effect depends on which part of the reed is scrapped; I am not going to enter in details, at least by now). The tone can be raised by cutting a small portion (a fraction of a millimeter, say 0.1 mm or so) from the reed's tip. Again, all actions which mutilate the reed must be made under the knowledge that this might completely destroy it.
To obtain a decent behavior on the second octave, the reed must be airtight. Try this: wet one of your fingers, place the bottom of the reed (where the open end of the metallic tube is) on this finger, and soak all the air inside the reed. A high-quality reed should stay closed a little while before opening with a ``pop'' sound. this behavior can be favored by putting some nail varnish on the reed's sides, where the two blades touch each other.
Occasionally you can see that a reed which has not been used for some time has some separation between the blades, mainly near the tip. This is not really a problem: it is just that the reed has been made in a humid weather, and it is too dry now. Just play it for fifteen or twenty minutes, and it will be in good shape again.
A final note: I have seen a tendency in some people to suck / wet reeds, in order to weaken them and make them easier to play. This is wrong, as it wets the reed suddenly and too much, resulting in a possible deformation. Any wooden piece subject to fast changes in humidity / temperature will suffer from internal forces which try to change its shape (usually in unpleasant ways). The speed at which new reeds absorb moisture is even too fast: the recommended way to break in new reeds is to play them only for a while (starting with some 10-15 minutes) and then increment this gradually. This ensures that the reed absorbs moisture evenly and that it ``gets used'' to being wet and dry.
Reed stopping is sometimes caused by excessive moisture. Rolling the drone reed between your hands four or five times solves the problem sometimes.
Otherwise, your task is to convince the drone reed tongue to stay apart from the reed body. This can be done just by pulling apart the loose end the of the tongue while pressing the opposite end with a finger (to avoid completely separating the tongue from the body of the reed). Hold it for ten seconds, then try to play again. If the reed still stops, repeat the above procedure, and use a match to apply a heat to the base of the tongue. Be careful: burning the reed is not your goal! Not even toast the reed: only heat it. Another common trick is to carefully pull the tongue and insert a hair or a very thin piece of thread near the base of the tongue.
If the reed is too unstable (its tone changes too easily under pressure variations) a drop of wax on the tip of the tongue helps to maintain stability. It also lowers the tone of the reed (and, therefore, of the drone), since it adds more weight to the tongue (thus making it more difficult to move), without increasing its stiffness. If you do not have wax, anything which can be easily stick and will no go away when the reed starts to vibrate (for example, nail varnish) will make it.
Scrapping the tip of the tongue will make the reed to vibrate faster: the reed it is lighter now, and the tip of the tongue does not contribute much to the rigidness of the reed. Scrapping the base of the tongue will make it more supple, so it will have a less strong desire of returning to its original position, thus lowering its frequency -- but this also weakens the reed, and makes it more prone to stop vibrating under an increasing pressure.