Need some help troubleshooting-

I have a replacement BCG I wanted to test to make sure its GTG. This is a good quality auto carrier and bolt.

Decided there was no better way than shooting my upper and new BCG on a member's transferable Colt lower.

With my upper on, the bolt would not drop on a loaded mag- it would drop as if in molasses/slow motion over the course of 15 seconds, then needing the FA to seat.
Would FTFeed if fired.

It did this with both GI mags and Pmags.
It didn't do this with his bolt in my upper on his lower.
It didn't do this in my upper on my lower.
My upper has 2k+ rounds with relatively no issues.
Lots of lube was tried.

Can y'all think of anything that could have been out of spec with his lower or my bolt that can cause this? I didn't see anything rubbing.

Bump

It's rubbing somewhere. No way around that. Push pin holes ar out of spec on M16 I'd guess. 

What brand upper? It may be the specs of your upper & BCG are on the "high normal" end & rubbing.

ETA: is the bolt lock engaging?

Amazed someone would let you "test" or troubleshoot your stuff on his transferrable lower.  I'm a nice guy, but no way in hell would I let that happen with my m16.

If the upper worked before hand on one of your lowers, it's the BCG.  The odds of his lower being out of spec enough to cause that are pretty low.  The BCG is getting hung up somewhere and that's the big change here