I have a carry forward to a parent on my sons tax return in the amount of $2400 in the student transfer tuition.

when i attempt to carry it over it says not available and 32400 shows $0 on both my wife and my return.  i have 3 kids in university and we haven't been able to carry over the tuition amount and i'm not sure why.   when i go into tuition transfer it states maximum transferable is 0