Your DEF system isn't operating efficiently enough. Possible causes:
-Clogged/dirty injector: remove and steam clean w/ an inexpensive handheld steam cleaner
-Old or contaminated DEF fluid: Empty and refill tank with new, good DEF
-Bad DEF pump: Not enough pressure to injector (after cleaning)
-Exhaust leaks: Large leaks upstream of SCR
The most likely causes are dirty injector and/or old DEF that's been sitting in the tank too long.