King Henry sees that you are not a reliable vassal to have in control of so great a holding as the duchy of Normandy. He decides to remove you. Ignoring a summons to attend the royal court will only brand you as further disobedient, making his case against you the more strong. Defying Henry from within your dominions will only delay the inevitable.
Flanders decides that you are too risky, and saves his daughter for someone else.
Finally king Henry is strong enough to move into Normandy with Anjou, as well as with Flemish auxiliaries and troops from your other neighbors of Ponthieu and Boulogne. At best, you will, upon being defeated, be allowed to live in permanent exile in England with king Edward, your loving cousin. History knows you as William the (unwise) Bastard.