Problem with FX magazine

I have been shooting with my FX Maverick .22 since December. Really happy with the gun but I have been having problems with the magazines jamming/stopping after about 6 or 8 shots. I first noticed it while iguana hunting- whenever I cocked the gun too fast for a quick second shot it would always jam. Cocking the gun very slowly has prevented this from happening most of the time but now it happens even when slow cocking. Probably seeing this problem with about half of the loaded magazines. I have 4 magazines that I keep loaded - I usually shoot barn rats nightly.