how to loosen immobile pump cap on bosch excell 1400 washing machine?

Bosch 1400 exxcel; has given me a "f04" error message- meaning pump needs unclogged. I have managed to empty extra fluid from drain hose, but have not been able to remove the pump cap. I am uncertain but think that the pump cap seal is seated wrongly. having managed to rotate approx 90degrees has helped it leak a little but is still largely immobile. any suggestions to help move cap without breaking handle?