If the snap aspect wasn't present, and you were only having difficulty flying straight in a loop, I'd say the rudder wasn't trimmed correctly...Wit the snap being present, I agree with the others that you probably have too much throw in the elevator halves....

Perhaps you need to trim the rudder also?

My test for rudder trim is exactly what you did with the loop....Fly a loop, making correction with the rudder....If it won't fly a loop with just elevator (assuming no wind), then I correct the rudder...With wind, I am aware that rudder is required to fly the plane through the loop straight.

I am far from an expert....I'm more of a beginner....I've had my edge for only twenty five flights or so.