Those definitely aren't Zippyy sticks. We ordered a bunch of that model from our supplier a few years back, and they didn't re-center properly. I shelved them until I could figure out a mod to improve them and never could.
Connect a multi-meter directly to the switch contacts and turn on the audible continuity test. If the stick isn't breaking the switch connection when you release it, or doing it very slowly, then that is the issue. About the only thing you can do in that case, is to replace the switches with some which have a larger actuation force, as it may help to re-center the stick.