First gun. Slide wont lock

[reagan2005]

I bought my first gun, a few days ago. Its a stoeger cougar 9mm, i took it to the range today and it worked great. i put a 50 round box of federal eagle through it, a 50 round box of magtech through it, and 15 rounds of hornady tap through it, and it didnt jam or misfire. i bought the gun new never fired... but it was the display gun, since they didnt have anymore in stock, but for 370 i didnt argue.

The only problem I had with it today was, when i went empty the slide wouldn't lock back. One of the range instructors told me, that it will happen i just have to break in the gun.

So what do you think it might be?

 

[modifiedbrowning]

Weak mag spring not pushing the follower up enough to engage the slide lock.