Why does hydraulic jack for main engine cylinder head gets stuck while opening?

If you are using the jacks to undo the head nuts you must back the jack off a full turn, pump the jacks up to pressure and you should then be able to back the nut off less than 1 full turn. If the nut is brought up to a stop against the jack and tension is released it will bind.

On tightening down you can screw the jack down onto the nut without backing off, as when you tension you turn the nut down and away from the jack.