4 low lock just blinking on/off when engaged

[06-H3]

Got home from the holiday road trip last night to a buried driveway. Had to get off the road so to be safe I was going to climb the mound left behind by the plow using 4 low. After shifting into netural and hitting the button, all it did was blink on and off. I've used 4 low many a time and this is a first. I was able to get in using 4 high lock with no problem, but I'm curious as to what the issue is causing 4 low to just blink on and off?