I had this very issue with my vsr. The fix is actually pretty simple. As the bb moves up there's an almost cup shaped guide that makes is shift forward and hold in place. In mine the bb was falling back slightly and the bb behind was pushing half into the chamber so they were jamming very effectively until I released the mag and apply a little pressure as the bolt shifted forwards.
Basically the fix is to simply deepen te cup slightly so the bb doesn't shift as it hits the guide holder. How did I achieve this? pushed a Phillips screwdriver into the cup shape and applied pressure as I twisted it to hollow out the hole slightly. I've done this on 7 vsr clones that have the same issue and none have had a problem since
It's worth mentioning that all of em had the laylax aero pss10 hop unit in them.
I guess all ur really doing is making the bbs shift slightly further into the chamber.