OK, I just searched for and found your thread...Having just experienced what brunbjoern was dealing with.
This is a simple problem with what should be an easy solution. What is happening is the actual button which connects to the pot-post is sliding down along the shaft and eliminating the range of movement required by the push-throw. What you need to do is go in behind the clear housing bucket through the access hole with a slender flat-edged tool and work the button back up off the shaft...and you may as well take it all the way off and shorten the sleeve on the button (or add a small ball bearing inside) so this cannot happen again, and re-install the button on the shaft. And I might add, the button did NOT want to come off of that shaft easily... PATIENCE..will be required. Once I got the button off, I realized the spring pot switch worked just fine.