My experience with WN online-check in (as well as Delta for the next step of printing BPs) is that if I'm using a web browser other than IE, the check-in system many times doesn't work and says online check-in not allowed, blah blah. Switching to my IE web browser has then allowed me to then check-in, no problems.

Maybe WN's check-in software is geared for IE, and other browsers produce glitches.