I've got 5 factory mags for my P238. 3 will cause the slide to lock back, and 2 won't. I've swapped mag springs around, and that doesn't help. I've also replace the slide stop spring. I noticed that the slide stop barely engages. I'm just wondering if others with a P238 see the slide lock fully engage, or does it look like this? Wondering if I've got 2 bad mags, or they are just a symptom of something else wrong with the gun. I've got an RMA to send it back to Sig, just hate to see the gun gone for 4-5 weeks.

It is possible you put the slide stop spring in upside down/backwards.  I made the mistake on my 938.  Field strip and try and switch it up.

One end of it goes into a little hole in the frame at almost a right angle, so there is only one way it can go in. Also, I watched how to do it on youtube, so now I'm an expert gunsmith.

When I look at it with the slide off, it doesn't look like the spring is long enough to engage the slide stop. It looks like the follower on the magazine has to do all of the work. This makes me wonder what the purpose of that little spring is, and why I swapped a spring from a working mag to a non-working mag. Both the new and old slide stop spring are the same length, and look like the same shape.

looks more like a problem with the slide stop/spring than magazines

Agreed. Think it is unlikely to have 2 bad and 3 under performing magazines. The thing that is puzzling is that it looks like both slide stop springs I have are too short to engage the slide stop. Guess I'll be sending it back to Sig to let them have a look at it.

My wife's P238 had that exact same problem, in fact your photo could have been my photo.

It took two trips back to Sig to get it fixed.  Send the picture with it and you should get it fixed in one.

I just checked mine.  With a factory magazine mine comes up just a hair higher than yours, almost exactly in the middle of the slot.

Hoping to avoid two trips. Do you recall how long it took them to turn it around?

One of the guys in my HTF posted this, which is what I would expect to see