I recently purchased a new Sig P229 SAS with 15 round magazines, and even with a magazine loader, I can only get 14 of the advertised 15 rounds loaded. Any thoughts on what is going on??
PS: The magazines are marked 229-1....

Maybe check that the springs are in the mag tubes correctly? I don’t know...

That's strange. Yes, I've certainly seen 10-round restricted mags set up this way (so there's absolutely no chance of illegally loading an 11th round), but never heard of a 15-rounder acting this way.

Like the restricted 10-rounders, your mag will probably loosen up with normal use and allow the last round to load. Why they would do this I don't know. Maybe sharing the same followers?

Abnormal, especially using a loader.

I'd start by disassembling the mag.

1) Lay parts out on the bench the way they came out. Check orientation of spring. Also check to see if it looks normal. No damage

2) Check follower and mag lips for any burrs, rough spots, etc.

3) Check mag body for any anomalies.

4) I usually take some crocus cloth and run it over the feed lips. Or something like Flitz with a cleaning patch.Essentially, it's just a light polish.

5) Thoroughly clean everything. Different opinions on lubing, but I lightly lube and wipe down as dry as possible. You don't want excess lube remaining anywhere.

6) Run follower down/up mag body to ensure no binding.

Reassemble. If it still doesn't take the last round, call your vendor/manufacturer and get a replacement. Something's out of spec.

There are 2 manufacturers of P229-1 mags, Mecgar and Checkmate. Mecgar's are marked made in Italy.
The Checkmates have been reported to be very difficult to get the 15th round in.

Quick update:
I loaded 14 rounds and let the magazines sit for a week, then got the 15th round in. It didn't just go it, I had to wrestle it in, but it went. IMHO: These are really 14 round magazines, not 15 rounders. I believe Sig wanted to try to be competitive with other similar sized pistols.

Another thought, there are some places like Colorado I believe that limit to 15 rounds. Maybe they were intended for that market?

Took a newer P229 to the range yesterday. First time (has P229-1 magazines) and had a heck of a time getting the 15th round in the mags.

My normal P229 is an older one with the SCT magazines (can put 18 rounds in them). Never had a problem with the 18 rounds, but getting that 15th round in the -1 magazines was tough.

Gave up on one of the 3 magazines and just stuck to 14 rounds, hopefully it loosens up a bit over time.

Any of my Mec Gar 15 rounders work as advertised.