Hollow tends to mean too much material was taken out, holes are not configured well on the bottom (size or placement), or it may be something with your attachment point of your pedistal, or a far other option (usually a dead sound, not hollow) would be your sound board slipped while it was drying and the sound board is resting against the wall of the pot now.
It could also be a situation where your striker is too heavy, either in your stick or the head.
The short of it, it could be all kinds of stuff.