Please forgive me if this is an old topic, if you have gone over this time and time again but to me this is a very personal subject.
In my club one of my kohai has a problem with ukemi. My kohai fears injury from tobi ukemi and indeed will not take them. I have tried many things from simply increasing the difficulty of the ukemi until it is tobi to a ten step ukemi programme, from a wonderful article in the asian martial arts journal my a Mr Wolfe, everything has failed. The sitaution has gotten so bad that the student stepped of the mat during summer, pretending to be injured, so as not to be hurt. And indeed is often hurt due to disengaging to early from a technique. Please be aware before a host of "time is the answer" replies land on the forum my kohai is first kyu and will be taking the aikikai grade next summer, probably with a stranger unaware of the ukemi problem. It pains me that this students is very upset because of this situation. To conclude the problem is not technical which the student isn't lacking but mental, one of the exercisese tried required the student to leap over my arm. Fear is the only problem.