Anybody have the roller-style chain adjusters and have an issue with them not holding place when tightened down? One of mine seems to keep moving on its own while the machine is in use. I constantly have too much slack on that side, even with new chains. I adjust it to the proper setting and then a few miles later, it slowly moves and the chain slackens. Could it be the threads on the bolt are stripped? Is it possible to buy just one of these? I see them only in sets of four.