The problem began slowly over time, but only within the last year or so. At first the thumb lever would only occasionally pop back to center. Now it only occasionally does not pop back to center. Although I have disassembled the gun to the point of removing the trip I did not do that until recently and only after the problem became more regular.

Now, having said that when I have some time I will again disassemble the gun with a parts explosion at hand and carefully check that all parts are where they should be.

Thank you for your quick response, and rest assured that i will check what you have suggested. I am not so arrogant to presume that I am perfect, you might well be right.