vaillant gb221h combi?

Domestic hot water fine. When heating calls for heat, the divertor valve plunger pulls in to the valve very sluggishly and slowly and sometimes doesnt pull in at all. The pump is running fine and the system has been vented. If the plunger is manually assisted the microswith makes and the heating starts up and works fine. Sometimes the plunger sags back and breaks the microswitch which in turn shuts the heating down.

any help please?

Paul