It is fairly common for these units to be stubborn, since few ever use them. I think 90 weight sludge just gums up the works. I have never had one of mine not work, because I used them so much. My experience has been just using it off road a bit, engaging and dis-engaging will clear it up. Remember, once you put a strain on the locker on something like asphalt you may need to wait, or back up and unwind that torque to disengage, similar to the viscous coupling coming out of 4x4. But the squealing tires is the diff locked.