I have two identical Bushmaster postban lowers, model # XM15-E2S. One of them will accept Orlite magazines without a problem while the other one will not will not accept the Orlites at all. I have three Orlites which will fit in the one lower  and the other will not even come close to accepting any of them. It seems like they will not go in far enough to lock in the catch. USGI's do not have a problem at all fitting in either lower. Does anybody have any reason what would be causing this problem.
Any suggestions would be appreciated.
Thanks.

It's the ridge on the side of the mags. The lower that they fit has a deeper bevel on the inside rim of the magwell. I got 2 orlites and one would and one wouldent. I just filed the ridge down a little.

Same with my bushmasters.  Check the series number of the orlites.  Its the bottom number in the circle on the mag. Anything higher than around 19 should be fine in either rifle.  I've noticed that the Orlites now in circulation tend to be the older lower series mags.  I had one I had to file down as well.  The higher series should work fine without any concerns.

The number in the circle on my mags have a 0 on top with a line through the center and 05 on the bottom. Would that indicate an older model mag?

Trade off the Orlite mags - The USGI mags are the only ones I completely trust.

I was not impressed with Orlites I got from CTD.  Half had cracked feedlips.  They don't trip the bolt catch well, and don't seem to snap in well.

On the other hand, I got some of the steel England marked mags from CMMG, and am very pleased with them.  They appear to be identical to USGI's, including green followers, in all respects but for steel instead of aluminum body.

I have 3 Orlites that I got off ebay for pretty cheap.  When they would lock into place I realized why they were so cheap.  I didn't want to put them back on ebay, (didn't want to knowingly auction off defective mechandise).  I didn't throw them away yet though, hoping there might be some solution.  Low and behold, this thread gives me something to try.  Thanks pilot for the post, and everyone else for the info!