Sorry, normally on AGF but looks good here. Where is sales section

Q. Bought an MP40 pistol last year. Mag was a pain. Zig Zag... cr2p. Returned. Now have a 2nd hand MP5K. BB. Like it but the thing only fires 4-5 then stops feeding. Have increased tension in mag (2 x 13a fuses).. have 1/2 loaded only. Still the same. Only paid £100, so a new £25 mag is a leap of faith. (Did the same withe the Mp40). Is there a 'trick' here or are they all just rubbish? Don't know how old it is, so could be the spring? (Thought the extra force by the fuses might cure that - didn't. Give up? All the same? Hang it on the wall? TIA