The dehedral will pull a trainer down very quickly inverted. I would suggest you start your roll after a gentle climb, say 20-25 degrees and let it roll around completely. Dont worry about rudder or elevator correction just yet. It might help if you increase the aileron deflection to speed up your maneuver. It really sounds like you are rolling too slowly.

Once you can roll exiting the same elevation as you started, you can practice feeding in down elevator while inverted.