Sounds like the common unscrewed ejector rod problem.
Try holding the cylinder while grasping the knurled portion of the ejector; turn it clockwise (I believe that all the Mountain Guns have left-hand threads on the ejectors) - it will probably unscrew. If so, you've found your problem.
The fix is to put a small drop of Loctite '222', which is a removeable version designed for fine threads, on the threaded portion of the ejector and firmly tightening with a special wrench.
Hope this helps, and do report back with your findings.