For a flat bottom wing trainer thats normal, another thing is quite often planes are set up to climb slightly (safty blanket) i found that seting the plane to decend slightly at about 1/2 throttle realy helped with inverted flight, probably due to elevator deflection.

i've found an easier way to practice inverted (rather than roll in) is to pull a loop and 1/2 way round (ie. inverted) start to push the elevator stick, you can also practise doing rolls and push elevator as you roll to try and get nice axial rolls.