I have 3 P99 mags all of which physically look identical.  One came with the gun and 2 I bought from a dealer online that are stamped LE/Military use (pre-bans).

Well, when I put each empty mag in, and pull the slide back 2 of the 3 work fine.  The 3rd (one of the 2 I bought online) is now not locking the slide all the time, and when it does "lock" it's more of a partial lock.. a slight nudge to the slide and it'll come off the slide lock.

Now, to resolve this problem I tried putting my hand on the bottom of the mag to hold it in securely.  This seemed to fix it.  The slide lock would fully engage.

Now, here's the stinker... I did not drop the gun or mag at any time, they all LOOK exactly the same.  I examined them all thoroughly.  I can't think of what has happened but something obviously did.  It happened after my recent trip to the range.  Got home and realized the slide wasn't locking properly on the one mag.  Yet it works FINE on the other 2 mags which leads me to believe something is wrong with that mag...

but WHAT?!

Swap the floorplates on the bad one with one of the good ones and see if that does anything.  Maybe the floorplate is cracked and effecting the spring tension of the mag spring.  If the same mag still doesn't work, then swap the floorplates back and then swap out the mag springs.  Do the same with the followers if that doesn't change things.  The idea is to isolate the problem to a single part of the mag.  Change only one part at a time.

S&W has floorplates, and may even replace them for free if they're cracked or something.  They'll have all the other mag parts too once you figure out what's wrong.  Wolff may have springs, I've never checked.

Also is your's a .40?  It seems the .40's have floorplate problems, especially on older mags of the "marked" vintage.  The 9's don't seem to have any floorplate problems.  

Yeah it's a 40.  I'll give your suggestion a try tomorrow.  See if I can figure out what's going on here.  Many thanks for the advice.

I'm betting it's the floorplate.

The .40's had a tendency to crack.  A cracked floorplate would do everything that your's does.  The mag spring isn't exerting as much force on the slide lock, since the floorplate isn't holding the spring tightly.  When you held the mag in manually, you were pushing on the floorplate, and it fuctioned like it had a good one.  It also explains why just that one mag is malfunctioning.

S&W will probably just send you free floorplates if you call and ask them for replacements.  

As another alternative, SW99 floorplates will also fit, and there hasn't been anyone stating they've had problems with those.  They just don't have the "Walther" banner, and are marked "S&W".

Well I just finished messing with them.  Currently I've got the "malfunctioning" spring setup and loader on the original floor plate and mag casing.  And then on the "malfunctioning" mag/floorplate I've got the original spring setup and loader installed.  I just swapped em.

I've tested racking the slide back about 25 times on each now and they both work.  Why?  I don't know but I tested it plenty of times.  It works so I won't bother SW with getting a floor plate but if I have probs in the future I'll make sure I do.

Pretty wierd but a simple switch seems to have fixed it.  Thanks for the suggestions guys.

I had a slide lock problem on one mag when I modified some Beretta mags to work. It was the small shelf on the follower that engages the slide lock mechanism (if that's the proper term). The Beretta mags had a pot metal follower and that little shelf was rounded rather than flat. I also may have cut the notch slightly high. I remedied this by putting a plastic follower out of one of my 10 round mags in the 92F mag. I can't imagine a factory Walther mag having the notch slightly out of spec but check the follower for wear on that flat spot that engages the slide lock.

I had a floor plate on my 40cal break. The wings attach it to the mag body broke off spewing all of the rounds and the spring on the ground. Walther was easy to deal with. I called them, explained my problem and they sent me 4 floor plates free of charge.