So I am aware of downloading standard capacity 30 round mags to 28 to ensure that it will seat and lock on a closed bolt.

However, I can't even seem to get a 30rnd PMag loaded to only 15 rounds to seat and lock in place.

It is NOT an issue of applying enough force, I have tried Pushing, Slamming, Tapping, Slapping, everything and it should just not require that much force.

Weird thing is this issue has just recently popped up after a pretty long day at the range as I could seat close to full mags on a closed bolt in the past.

Mags: Gen2 Pmags
Ammo: Wolf Steel Case 55grain
Rifle: Smith and Wesson M&P Sport2

This is a pretty serious issue because basically the rifle is not functioning as designed with out the ability to reload on a closed bolt.

You know that slop between the upper and lower  some complain about... that slop  aids in letting a closed bolt and full mag  lock and load

This happened to me with my brand new noveske sbr. Only lancers would fit.

some "slop" isn't going to stop a full magazine from being inserted and lock on a  closed bolt. I  remember being issued a M16A1 that had so much slop between the upper and lower receiver that it rattled and that never caused a full magazine from being inserted on a closed bolt. I have one AR15 that has a bit of   "slop" between the upper and lower and that also doesn't cause a full magazine (M16 mag)  from being inserted and locked.

Patriotnations766,try another manufacturer's mag  and see if that seats and lock. if it does something is wrong with the PMAG, then if you want to take the time and trouble, contact MAGPUL and see if they will exchange it. if that mag has the same problem, contact S&W for warranty work

I've read the OP post. he didn't write anything about it working before now it doesn't.  otherwise fadedsun would have caught that.

anyway, I still stand by my advice to get another manufacturers magazine, load it up and see if that seats and locks in the rifle. .

OP,
If it worked before and just started after a long day, then I would start by making sure everything is clean and lubed properly.  If it's a new-ish rifle then it just might be sensitive to being dirty and needing some break in.  Pay special attention to the magazine catch and the bolt release and the contact surfaces of the magazine feed lips and the upper receiver where they mate.
Also, some rifles are just magazine sensitive and some not.  I read about more tight fit issues with P Mags than any other, although I personally have had no problems with them.