Is this Duet new?
If so it will take some good amount of time to run in completely. Mine took around 300 hrs. I don't know what it sounds like right out of the box... I usually forgoe that issue by plugging power cables, line filters, etc., into some household item like a fan, fridge, etc.
Tight outlets? yeah, they're tight... but standard NEMA plugs should have little trouble getting in there. the last bit of travel in the receptacle does require some extra effort though... and yes, different cords will seem to be more stiff to seat than others....
I'd take the one which does go in and plug it into the other outlets some to loosen them up a mite.