Usually hub seals but the regulator/solenoid on the fender has been known to fail also. Easy to check the system with a $20 handheld vacuum pump/tester (most parts store will loan one for free). The hubs should get a pulse of vacuum that lasts about 30 seconds when switched on and again when switched off. One pulse should be 6-7 and the other 15. If that is happening then the problem is in the hubs, you should be able to hook the tester to them and pump up to 15 vacuum pumping pretty fast by hand. If you can’t draw a vacuum then this leak is likely the problem. I have also disassembled my hub locks and cleaned and lubed them once to fix mine. Also sometimes when there is a problem the hubs will not lock but other times they will not unlock. It is important to periodically check to see if the are locking and unlocking right. I have my regulator/solenoid unplugged because I like to use 4x4 low for backing trailers but still want my wheels free to steer sharply.