Most of my steel surplus mags fit fine, but I'm having trouble with 1-2 steel mags, Bulgarian circle 10 waffle mags, and Magpul Pmags. They are hard to insert and even harder to remove. Any idea what part of the mag is causing the hangups?

Check your magwell for bumps and burrs. Especially on rifles like the WASR10 which were converted from single-stack mags.

If you find nothing there, then try putting some sharpie or crayon/wax on the mags, attempt to insert them, and see where it's rubbed off.

This part right here^^^

Also, when you say difficult to insert/remove, do you mean "insert/remove" or "lock/unlock"? If you're getting interference before the rear locking lug gets near the receiver, you're mag is likely too wide (or magwell too small or uneven). If the problem is in getting the rear lug to lock in or or in getting the rifle to release the rear lug, then you may need to relieve the underside of the lug. Regardless, before doing anything, mark up the mag with a crayon as suggested above.

I would make adjustments to a cheap rifle before I did so to quality mags. The mags are in spec. The mag catch isn't.

All around terrible advice. You're assuming the rifle is cheap and out of spec. You're also assuming the mags are in spec (as if there was such a thing). You also seem to have completely overlooked his statement that all of his other mags work well, which means that modifying the rifle to fit this small number of recalcitrant mags could result in the majority of his mags not working. Always modify the part to fit the rifle, not the rifle to fit the part, unless nothing fits the rifle properly.

There is no mil-spec when it comes to Ak wells.

But filing away on a mag well is not the best thing to do IMO.  I always file the rear tab on a mag to fit it.

Having the mag catch adjusted slightly (or doing it yourself if you're confident enough) if the right answer. Bulgarian Circle 10 mags fit right in some rifles whe. It comes to mag well width, but the OP isn't having a width issue. His mag catch lever is likely just a few thousandths of an inch too high. I had a SAR-1 over a decade ago with this issue. It was a simple fix and the rifle worked with all mags after the adjustment. Not just some. While I wouldn't adjust a rifle for crap Magpul AK mags, if Bulgarian and other quality (non-Korean) steel mags are not locking in...I would.