Re: Ukemi problems
The post above about starting certain novices with a pronounced fear of falling (and you will be able to tell them almost right away) from not only down on one knee, but all the way down on both knees, hand-arm thrust back towards the opposite foot with head turned and tucked to allow the shoulder to actually start out ON the floor is an excellent tool we've used for some time with folks with falling fear.
There is no "fall" per se in this practice drill, yet they get the concept of the ball, and rolling as a ball, and they feel the line the body takes across the ground, yet there is no impact. Even starting on a single knee, if the lead arm collapses there is the possibility of a shoulder ding, so keep an eye out for that, too.
Once the student is doing the shoulder down rolls, and they've gotten to the point where they are obviously bored with them (might take 3 minutes or 3 weeks), then bring them up to a knee start, with the hands on the ground (hands, plural), then to a more fluid feeling hand edge or palm (depending on the theory your dojo uses to teach zempo kaiten ukemi), and then a transition to standing rolls is as simple (right...) as saying, "What you are doing kneeling, just do it standing up.
Some folks will nail it the first time, others might crash and burn. C&B causes the loss of students, so try to avoid it.