bolt issues

[camden]

Now this is really foxing me: I am using the original bolt (reinforced spring inside as the old one broke). With a fresh mag, it fires well for the first few shots and then performance drops hugely and it can't empty the mag. Using my mate's original bolt, (and using the same rifle and same magazine as before!!), it fires absolutely fine and empties the mag reliably.

I have since changed to a light weight bolt (had one lying around so worth a try) and changed the pistol head/seal as well; with the same result. Anyone clever out there can point me in the right direction as I am really missing using my MWS?

 

Thanks in advance!