if people are having trouble I take all of the height out of the roll that I can.
I have them go on the floor, with their knees and their hands on the mat.
I then get them to push their left knee away with their right arm. (or left.. the arm pushes the leg backwards extending it)
This will put their shoulder directly on the mat. they can then use the left leg to kick over the top.
we then slowly build the height up. it only usually takes a couple of sessions before they have enough confidence to roll from standing.
hope my explanation was clear