What's happening is that you're losing speed up the first half of the loop and the model is slowing down enough that with full up elevator it's literally stalling and then snap rolling out of the loop.

As mentioned already you want to ease up on the elevator over the top or get more speed before entering the loop.

Loops done with constant full up elevator tend to be rather egg shaped anyway due to the speed issue. So to make them a proper round shape you need to ease up on the elevator over the top then apply full elevator on the last part of the downside. Precision aerobatics pilots are using a constantly varying amount of elevator to generate truly circular loops for this reason.

And if your model has a lot of elevator deflection it's likely that you're making it loop so tightly that it's near the edge of a G loaded stall for much of the loop. Try getting up a bit more speed and then do a loop using a bit less then full up so you have something to work with for altering the curve at various points around the loop as the speed alters. The loops will be more round and at the same time less likely to do any high speed/high G stalls.