Hi,I have the same problem here in the summer. I bought an Australian made tuneable becasue my Irish made one just hated the climate over here (played beautifully in Ireland.. still plays beautifully over here when it rains!!). In the winter and cooler weather it plays fine..... but in hot weather I have to loosen all the screws completely, and give it lots of water...... I don't know if you can loosen the skin permanently..... but to honest I'd wait a few seasons anyway .... it may play just fine at other times of the year.....
here's an earlier thread...
bodhran making - a tale of woe
slainte
alison