It it possible you were hitting the slide release?
Also - can you isolate the failure to lock back to just 1 mag?
I love my P99. I like many guns and have owned many over the years. However - the P99 A/S fullsize is the most accurate gun for me. It is actually quite maddening. I have my accuracy with all my other guns, which is generally about the same. Then I have my P99 accuracy - I cannot shoot any other gun as well on paper consistently.
While I am glad I found the P99, it's irritating sometimes that I cannot shoot as well with any other gun I try
I don't shoot my P99 as much anymore, as it's almost boring to do. I know what I can do with the gun. I shot a 250/250 on my CHL renewal in 2007 with it. First time for that.