Hello - there may be a number of reasons for this and I'll leave the pipemakers to argue over them all
But what is happening is that drone reed tongue is clamping shut. The first step is to try is flicking the tongue to try and reset it a little higher. If you've not been inside your pipes before and had someone show you how to do this then it is somewhat risky (and not a little daunting).
You don't say whereabouts you are in the World but I would strongly advise seeking out your nearest friendly piper or piping group to get someone to demonstrate this sort of thing. Failing that there a couple of books which go into a fair bit of detail on reed construction (which is half maintenance anyway).
- Colin Ross's Reedmaking booklet (available from Julia:
http://www.northumbrianpipers.org.uk/pipersforum/memberlist.php?mode=viewprofile&u=56 - John Liestman's book is an excellent resource (and free):
http://www.liestman.com/#tutorcheers
Rob