Earlier this year I bought my first banjo with a hide head. I'm really enjoying it but have noticed that when I first pick up my banjo all of the strings have gone flat. So I tune it, but then after I've played for a bit, all of the strings have gone sharp. None of my other banjos have this issue. I live in an area that is fairly warm and humid this time of year, so the AC is always on and the windows are shut. I've been paying attention and it does not seem to be related to the level of the outside humidity.
Is this normal with hide heads?