Thats how it goes for corners and WR sometimes. Only a few are good old like Green and Bailey, Woodson, Tim Brown etc etc. Others are one of the better ones in the league for 4-5 yrs but can't maintain that like the great players.
Nmandi may just be one of those mortal players with a 3-5 yr peak period that is now just over with. But I still think he's probably a starter in this league.