I think that there can be some slight differences in these adapters - even the Leica ones! Tolerances for Leica lens mounts must be very small. Interestingly I too once owned a Summarit with a Leitz adapter and found it was a quite difficult to mount on my Leica M camera bodies.
I eventually got the lens and the mount separated (it seems that a previous owner may have loktighted them together - which was another unrelated issue) and tried the adapter with other lenses.
From memory (I have since sold it) the adapter also was somewhat tight to mount when other lenses were in place. This makes sense of course as the lens should not make a difference.
And what is more, the problem persisted with each M body I tried it on (2 M3s and an M4P). So the issue clearly must be with tolerances of (or damage to) the adapter. I never noticed any damage (although I could not discount that.)
Interestingly I had other Leica adapters including another 50mm one, all of which were much less difficult to mount than this one. At the time I distinctly recall that I noted differences in appearance between them - principally that the chroming on the problematic adapter was much more shiny than the satin chrome on the others (and my impression was maybe it was a little thicker) than on all of the others.
Could this be the issue with yours????????