The piper not only has no direct mechanical control of the melody reed, but can't even modulate air pressure to compensate for pitch problems because a change in the pressure delivered to the chanter reed also changes the pressure delivered to the drone reeds, and the instrument will become out of tune with itself. So, a bagpipe chanter reed must work correctly without being manipulated, directly or indirectly, during play. It might seem that once adjusted properly a reed, being nicely protected deep within the instrument, would function well for a long time. Unfortunately this is rarely the case, in large part because of unavoidable fluctuations of moisture. As a reed becomes damp its characteristics and behavior change.
Temperature also plays a role.
The result is that a reed may perform well early in a playing session and then suddenly refuse to play some notes, or play them grossly out of tune, or make rude gurgling sounds, or squeal violently. For example at an indoor wedding when the piper has warmed up outdoors, reed stability may collapse quickly when the pipe is brought inside. The reed may also require adjustment from one day to the next, depending on changes in weather, altitude, temperature and, it sometimes seems, for no reason at all. Finally, the constant wetting and drying shortens the life of reeds, which are subject to splitting or simply becoming worn out. Trouble, when it strikes, often leaves the piper with no choice but to stand red-faced in abrupt, deafening silence.