Basses are so huge that a slight change in humidity might make the wood expand along the width by even 3/4". It's nut's. It's no wonder that when my bass moved from Utah to Portland, the wood swelled, popping open a bottom seam. Because I didn't fix it right away, the wood warped to a point that if you pushed the back plate in to touch the ribs, the curves didn't match. The only way to fix that, really, is to take the back and front off and reshape the ribs. YEAH, RIGHT. Not happening.
So I glued it back together and I'm just accepting the fact that that bottom bout is slightly warped. Wood is wood, and I can live with the fact that its an organic substances that changes shapes, like glass panes that get thicker at the bottom over time.
Note on hide glue: you should never use it to fill a gap. It's just a thin application that is absorbed into the wood for wood-on-wood contact.
Also, as I was working on this the glue started gelling, which is not great. I was worried after all that careful clamping that it wouldn't hold, but Elmer had the idea to just reheat the seam with a blow dryer. So leaving the clamp in place, I blow-dried the seam and watched as the glue remelted with some melting and beading out the seam. That's good. You don't want more glue in there than you need.
I like to clean the excess glue that's pressed out right away before it dries with Q-tips and/or a paper towel dampened, not sopping, with HOT water from the glue pot/double boiler or the teakettle in the break room or on the stove. It's not great to try to chip away hide glue once it's dry. You'll notice that when you scrape old glue out of an open seam, you have to be careful not to chip or splinter off some of the wood while you're at it. Hide glue just adheres so well. So clean it off before it dries.