I found out a couple of things that I started doing and haven't had that problem since (thanks to lots of reading on here!).
First, if you're putting anything but a layer of buttercream between layers, make the dam of buttercream to hold it it BUT don't make the dam right at the edge of the cake...set it back about 1/2 an inch. That way when the layer on top settles, it can squish out and not look weird.
Second, don't try to stack too many layers without a support in between. I'm not a professional or anything so this is just what I've found...two regular layers, even torted can support themselves, but more than that (again, in my opinion) starts to be too heavy.
Thirds, once you stack the cakes (only putting in the dam and the filling) let it sit for a least a few hours, if not overnight so that everything can settle. Then, when you go to frost it fill in the "crack" between layers first and then frost like normal (top, then sides, then smooth it out).
Hope that helps, good luck!!