Don't quote me on this but i heard that the P-mag will hold upto 31 or 32 rnds. the reason is to give extra spring room for the dust cover which also releases the tension off the feed ramps on the mag. Some guys just fill the mag up until no more rounds will go in which makes seating the mag extremely hard. Check that and let me know if it is true. I do not have any P-mags so I can not check that theory
Make sure you only and always load the mags to 28 rounds and smear chalk on the portion on the mag that enters the magwell. Insert the mag and remove and you will see the high points on the mag.
The other issue could very well be your lower is not to spec. P-Mags will work in a spec lower. Your metal mags work because they are a few thousands thinner. Some have had issues with the band that seperates the top of the mag from the bottom portion that sticks out of the rifle. This band can be sanded down or or you can buy E-Mags that will fit, however this comes down to the same issue that original P-mags fit in spec lowers.
No reason not to load 30. I've loaded all of my Pmags to 30 rounds and never had a problem.
There is an excellent reason to load to only 28, it allows compression of the spring when performing bolt forward reloads. It guarantees the magazine will insert and lock under stress and accounts for variances in the orientation of the rounds when they are loaded.
If you are worried about two less rounds in the magazine, those two won't solve anything that a reload would not solve. You should not be shooting to bolt lock anyway.
Twenty-eight rounds in a magazine has been proven on the ranges of several high quality instructors as magazine loaded to 30 consistently end up on the ground during reloads as they fail to lock in place.
There is an excellent reason to load to only 28,
in a USGI mag 28 is a good idea.....PMags have accounted for for the extra needed room, load 'em to 30