Slowpoke has 3 base egg moves, with Tackle last. Moves must be pushed out before they can be re-added.
To push out 3 base moves to reapply Yawn, parents must have 3 shared level up moves, and Yawn last.
The parents only have 4 move slots each, and all 4 have been used. We cannot have ANY egg moves, as there are no more parent move slots (need 3 between them).
Working as intended.