Yesterday I was at the range all afternoon and had a problem with the slide stop on my G17.  Everytime my magazine would run dry, the slide would not stay locked back.  It happened with all three of the magazines I was using and it happened every time.  I ran about 200 rounds through it yesterday.  I had this happen one or two other times in the past, but not on every magazine like yesterday.  So what should I do?  Anyone ever had this happen to them?  Thanks in advance for your help.

Are you using the +2 floorplates?

NO - I am using regular 10 round mags.

A few suggestions...check your grip when shooting...it is entirely possible that you are pushing down on the slide catch with one or another of your thumbs...it does not take much pressure at all to prevent it from engaging. Remember, too, that just because you don't think you are doing this, is no proof that you are NOT doing it under recoil with live ammo. Maybe get someone to watch your hands when you shoot.

Try the gun with an empty mag and see if it will lock the slide back whan you manually pull it to the rear "smartly"...pretty hard. If so, check your grip again. If not, it could be that you have taken the mags apart for cleaning and reassembled them with the spring facing backwards, which will cause this, or less likely, that your mag springs need replacing.

Two other things that might cause this...an extremely dirty / unlubricated gun with lighter-loaded ammo such as standard WW "white box" ala WalMart, and / or a very limp hold with the last shot...but these do not sound like quite the problem you were having.

I hadn't ever thought about my grip possibly causing this.  I hadn't considered it, because I assume that I am still gripping the same and shooting the same.  Nevertheless, I will have someone watch me.  
The slide does stay back when I pull the slide backwith an empty mag.  And the gun was well cleaned and lubricated when I was out shooting.  
I was shooting some cheap Wal-Mart stuff.  The 115gr white box ammo - which I have shot before and never had the problem.  Thanks.

I purchased 4 10rnd glock mags w/new follower(flat front). they feed ok but don't engauge the slide stop after firing last rnd.I believe it's poor design. Don't have any prob with old followers or highcaps.

I was thinking about buying some hi-caps - even though they cost a fortune to see if they worked a little better. Thanks.