Have you called Force 10? I had good phone
support from them.
There is a part inside the valve that interacts with the thermocouple which might also need replacement. One other thing to try is:
Remove the knob, and hold in the shaft while you light. If it stays lit the problem is the hole in your knob is too deep (not pushing the shaft far enough in). I had that issue on one knob and it was a simple fix to insert a small item into the knob's hole to make it "shallower".
Edit: .. or try standing on one foot while you light it.... or tilt your head