Rear Differential Lock Won't Engage

I put my truck into 4 low today and tried to engage the rear diff lock. I had a-trac on and traction control off. I also had the truck in the low gear on the transmission. I clicked the button to engage the rear diff lock but the light on the dash just flashed at me and never turned solid. Am I missing something?

 

to get the locker engaged, be in 4 lo, then stop and go into neutral, then push locker button, then drive, forward, back, turn wheel back and forth, and maybe it will come on. if not, repeat......

 

Alright I'll give that a try.

 

the more you do it, the faster it will happen ........