I had a similar problem when teaching Teddi the "play dead" trick (partial "roll over")-- I was teaching him to lay down first, treat, then roll over by nudging him on his side and treat again. Eventually Teddi would only lay down and not roll over until a few minutes later of us staring at each other.
So I had to re-teach the trick. This time I used the treat to lure him to "down" position (no treat yet!) then lured him to roll over on his back (with a circular motion of my hand towards his belly). When he was on the correct position, I'd say "Bang!" (my "play dead" command) and treat. Did that 1000x and now he can play dead!
Roll over is similar-- use the treat to lure him to roll over (on his own, without touching him) and once he completes the roll, say "Roll over" and treat. Repeat 1000x!