I have two FALs, both 21". The DSA STG58 seems to work with just about any mag I stick in it. The Imbel fails to feed properly with some of the same mags.

The bullet doesn't make it all the way out of the mag and the bolt/BC just scrap up the side of the brass and gouge the side of the bullet.

Both have the same setting on the gas regulator and the Imbel works fine with the mags it likes. So what might be causing  it to not to work properly with certain mags? Thanks.

Bolt over base malfunction.  I'm having the same issue with my IMBEL with most magazines I've run through it so far.  It only happens when there are five or fewer rounds in the magazine, and usually on the second to the last cartridge.    Some of those magazines have worked fine in a Coonan and an FN (the others have only been used with this rifle).  An FN alloy magazine is the only one that has run flawlessly so far.  The others tried have been Steyr and IMI.  I'm not sure what the cause of it is.  Gas is set about where it should be.

I also only had 5 or less rounds in the mag when it happened each time

The lip height and shape of the magazine, internal dimensions of the magazine and the follower and spring pressure of the magazine are the likely differences.

This is not a new issue with FALs. Some years back, FALFiles member "ggiilliiee" posted a fix that involved matching the width and contour of the receiver rails to the magazine's feed lips. Naturally, a Dremel can be involved, but the mod is probably better done with file and 400 - 800 grit paper.
If you remove the top cover and insert a loaded magazine, you will see that the receiver rails hold the cartridge down well after the magazine's feed lips have released it. If you open the rails to more closely match the feed lip's width and contour, it allows smoother feeding and less battering of the bullet nose. Smoothing out the underside of the rails is also very helpful, and is sometimes all it takes.
The caveat is to go slowly, and do no modification of the rails forward of where the case mouths sit. Getting too aggressive will result in the bolt hanging up on the left rail when going forward on an empty magazine; it does not affect feeding from a loaded magazine.

Unfortunately, the original link to the mod is broken, and photobucket killed all the pictures. That is not as great of a loss as it may seem, since the original was written in ggiilliiee's rather bizarre, stream-of-consciousness style with frequent ejaculations and liberal use of somewhat random punctuation and capitalization. Laborious translation yields the above distillation, and a google image search for "ggiilliiee FAL receiver rail mod" will still yield previews of the now-defunct photobucket pix. Those preview pictures are fairly self-explanatory, and show pretty clearly what the whole thing is about.

Might want to consider modifying the mag lips, rather than the receiver.  Lots of variations in the width between mag lips, and there are tools designed to modify mag lips.  Find one that works good, then measure distance between mag lips and check against the ones that don't work so good.

Does the mag sit all the way up against the underside of the receiver?

Is the front of the bolt on the bottom rounded on the rifle that has issues?

I had this problem years ago on a build and I replaced the recoil springs with L1A1(longer) and it solved the problem.