I recently purchased a used Beretta 92 brigadier, i went to the range to test her out. On the last round the slide will not lock back, it does occasionally but most of the time it does not. If I pull the slide back with a mag in, the slide does lock back on it's own. The gun was used and came with one mag, so i picked up two others (Beretta made). The new mags make no difference and the slide still does not lock back. What could be wrong?