NEXUS users, along with Global Entry users, are Pre eligible. (I'm a NEXUS member and get Pre.) The PASSID on the back of your NEXUS card has to be in the Known Traveler Number section of the Secure Flight data on your reservation. Also, make sure that your first, middle and last names, along with the gender and DOB in Secure Flight, match exactly to your NEXUS card.

Your BP has to show "TSA PRE" (along with the 3 beeps at the BP scanner) to use the Pre line. Just flashing your NEXUS card without "TSA PRE" on your BP won't get you in the Pre line, but NEXUS cards (along with GE cards) are acceptable identification at all TSA checkpoints.