Most trainers need the help of the rudder to complete a roll. Get the plane on a slight inclined attitude, and then push BOTH sticks one direction or the other (but not opposite directions). 1/4 way through the roll, start to give some down elevator (while still holding right or left aileron). when completely inverted, hold enough down elevator to keep the nose up. Then, roll out using the same technique (begin to slowly release the down elevator as you roll out, until you are flying upright again, and have neutral elevator again.