Well I would try garden hose flush of that discharge line!

Incidently my sump pump has a small hole thatrs been talked about, I
think its to avoid pump burn out if the exit gets blocked, remember
the flowing water cools the pump.

My pumps have had a form of that hole as well. Also, I snake my exhaust
lines. I have pictures of that too. My pipes had about a 1/2" coating of
red mud on them. Thats why I installed the cleanout. I need to snake
once a year. Too bad I didnt know this before I put up the drywall or I
would have put in a better elbow...