They have knurled wheel inside that dents into a plastic strip to allow infinite adjustment smoothly. If by chance you leave the lock lever locked and turn the wheel it can strip out that one point of the plastic strip. This is really hard to do and you'd have to be pretty heavy handed to have had that happen and not notice.
Secondly and most likely is that the set screws to adjust how tight the knurled wheel indents into the plastic and is pressing on the strip have come loose or fallen out. If I'm not mistaken they are a 4-40 thread and can be replaced pretty easily in a pinch. If they are still there, simply adjust them in a 1/8 turn at a time until the wheel engages and moves the sight up and down. If you get them a little tiny bit too snug and run the slider up and down a number of times it will reengage the knurling and make it run perfectly after a few cycles. Then blue loctite the screws so they don't come loose again.
When you get some time send it back to Black Gold and they will sort it out perfect. They probably have the best service of any company in the industry, but sometimes stuff does happen, unfortunately.
Cheers, Pete
