Martin AS 40 will not shut off

Older LP fireplace insert. Pilot lights readily with ignitor, burners adjust and burn normally. But when control dial is turned to "off" pilot will not go out. If I turn off supply and let it cool for a minute, then pilot gas flow is off. I have found that spring-loaded valve that is opened when lighting pilot is sticking open and not springing back to closed position when dial is set to "off". But when cool, it returns to closed like it should.

What is the problem?

Thanks for any help.