I have this too from time to time. The problem is the kneel. sometimes it just isn't comlpetely down, so while still hooked to catapult I simply lift the nose a bit and then kneel down, and make sure to keep the switch down at least for 5 seconds.
Once completely down you will notice the plane sort of locks itself with the deck (kind of like when you shut down engines on the deck and the planes snaps together with the ship). Then the catapult kicks in, so I would recommend to keep it at the mill power before you do that.