how high are we talking, is the bolt face clipping the mag, or pushing the round up out of the mag instead of chambering it. ive seen a couple of mags jaws spread by forcing the fifth round, this can lead to the round been held unsecurly, in some cases sitting at extreme angles nose up or down so that the bolt can skip over the round or it can literally jump out of the mag as soon as you attempt to chamber it. just out of interest is the problem with the mag that came with the rifle or a spare (could be one someone elses problem passed on).