I am still relatively new to the Land Rover Series IIA and have not used the four wheel drive since I bought it a year ago. It engaged and worked when I bought the car, but now a year later I tried it again and the yellow knob engage lever will not stay in the depressed (engaged) position. It jut pops back up when I push it down. I read the manuals and seem to recall it stayed down by pressing it down. Am I missing something here or did something break? Thanks in advance.