Cheap/non-contract MP5 mags generally suck. They are a waste of money.

Get POF or MKE/Zenith if you want in spec contract mags for a "fair" price. Or bend over and shell out the dollars for HK mags. Other than the name stamp, I can't tell a difference in performance between my HK, POF and MKE mags. If I had to be picky, I'd say the HK welds look slightly cleaner but maybe that's because I want them to look better due to the price I paid for them.