I have noticed that some people who have trouble taking ukemi from kaitenage also have trouble throwing.
I think it is a reluctance to give and receive. If nage firmly throws uke then there is the unconcious (perhaps) feeling that they will recieve like treatment as uke.
Recieving ukemi from kaitenage requires acceptance and trust. Both of these come from practice, patience and compassion.
I used to HATE it.... now I LOVE it.
Berkshire Hills Aikido