What could be preventing the slides from locking after last round fired?

I took my two carry guns to the range this morning for the first time (Walther PPS M2 and VP9sk). I’m getting intermittent slide locks after last round fired. Every round fired successfully and no stovepipe’s.I was first using 70 grain and thought that was the problem then switched to 114 and still intermittent. Any suggestions?? via /r/CCW https://ift.tt/3uGBpUT