Series 3 - yellow gear lever not engaging 4wd

New to land rovers so please excuse my stupid questions

My 4wd is not engaging when I depress the yellow lever and I am in Hi-box. To use 4wd I have to go into low box and then it works fine. Is this ruining my life.......No, not at all. But if it is a common problem with a straight forward fix I would like to correct it

All the connections from yellow lever to transfer box are in tact and moving freely so whatever is wrong appears to be gone wrong inside the transfer box. When I depress the yellow lever in the cab it does not stay depressed and instead comes back up straight away.

Am I missing something obvious that I should be doing when using the 4wd in Hi gear??

Change the knob.

This may sound stupid but are you pressing it all the way down.

Pressing down as far as it goes but no connection happening in the transfer box.

theres 3 shafts that poke out the front of output housing the center small diameter one is the one the yellow knob engages in its a pivot between the 2 outer shafts it sounds if its seized in the housing

Take the dust cap off the end of shafts and check dimensions against attached.

I may be wrong,,,but I think that when you use the red knob the engament of the 4WD shaft is positive (ie you shove it) but when you press the yellow know its shoved by a spring. Putting it from 4wd to 2wd compresses and latches the spring, the yellow knob unlatches it.. My guess is the shaft is stiff and the spring a bit weak.

Thanks for replies. I took up the cab floor for a better look at things this morning.
It looked to me (the novice eye) that the 4wd locking pin was riding high all the time and never actually dropping down fully. Also pressing down on the yellow gear did not seem to be doing anything - no engagement sound coming from the transfer box. So I took the connections and the dust cover off. Cleaned everything up and now it works. So I'd say the problem was mainly dirt & grease build up. Also need a new spring for the 4wd lever.