if you mean the 10 shot mag conversion they can have problems if not correctly fitted, the indexing system that turns the mag can get bent, the mags are not the best quality, i would suggest you take off and check all parts if every thing ok replace with bolt through then tighten holding screws, once it aligns good after you trial without pellets, use pellet lube and they are better with shorter domed pellets, good luck from the oldy
Thanks everyone will consider all the answers and work through them one by one including drilling woodwork and report back on conclusions/ solution
As it now seems to be one mag at fault would it be worth cleanining it up with very fine wire wool or getting it replaced? should be under warranty not quite 6 months old,