Ok time for me to help out here if you have been a little foolish and crammed diablos with abilities he does not need e.g. Sum mag abilities which are pointless on him as he is hp based fixed damage - something like enemy original full HP / 4 and then you have 1/4 of their full health which is the damage he will cause but never exceeding 9,999 damage. If you taught him sum mag forget them immediately as they are pointless same for all other "Support" GFs like carbuncle and cactuar if they don't start with it on the list it is not needed... Also Diablos for all his power should never be junctioned without a more prominent attack GF such as Shiva, Ifrit, Quezacotl but never Alexander - Holy and dark they hate each other like junctioning shiva and ifrit one lowers the others compatability then delete / forget the following moves only the following ones:

commands such as:

DRAW
MAGIC
ITEM
(not darkside one of the best moves in the game)

also once learnt GF HP + 40%

GF HP +10%

and possibly 20% if you get a royal crown and teach it 80% if I remember right

this will give you around 5 free slots to play with try putting a gamberler's spirit to him to teach him card then keep him separate from Quezacotl who has that ability when carding you have 2 people capable and one to attack

there this should sort out the problem and yes Albhed3 you are correct the abilities they learnt or didn't learn due to not enough space return to the list