Hard to diagnose with handling it, but my guess would be to remove the lever and put it on a granite flat or piece of flat glass and use a feeler gauge to see if it is bent. I'd also inspect the lever pivot to see if it has a burr that is allowing the lever to ride high on one side.
I can't see incorrect assembling causing the lever to rub, if anything it would be a sloppy fit. Something is out of wack to cause parts to rub.
Unfortunately more than one mountie has seen it's fair share of abuse. My 39a had a broken firing pin and missing cartridge guide so took some fiddling, hand fitting of the firing pin and thorough cleaning before it would feed and fire reliably.
