Collective pitch timing is the secret to doing nice axial rolls. When the heli blade disk is perpendicular to the ground you should be at 0 pitch. When inverted you should have a little negative pitch. Then back through 0 again when perpendicular before coming upright.

I practice by doing stationary rolls and when I do them while staying in the same place in the air the timing is right.

Try moving the collective slower through the range when doing a roll. I see a lot of new guys that are moving the stick too fast and end up with a barrel roll.