4wd won't engage single flash

Right I'm having a problem that I can't seem to find the solution too nor have I seen anyone else have this problem. 

2006 jimny with the button 4wd, I click it and I can hear the transfer box do the check by engaging but then it disengages it, I have manual locking hubs, I've blocked the vacume lines, I've bridge the cables together and nothing.. Still single flash of green 4wd light on the dash. 

The only way I'm able to get 4wd is if I lay under the jimny, get someone to press the 4wd button so then it engages on the transfer box brifley I can then unplug the power to it so as it can't disengage its self which work but it's not great.. With this I can't get into 4wd low bc soon as its re-plugged back in it'll come out of 4wd again. 

I'm at a total lost end with it and considering getting a stick shift tranfor box out of an older jimny. 

Any help would be much appreciated. 
​

The switch is occasionally known to fail. Replacement is simple enough but there is a tiny ball behind it that acts as a detent and is very easily lost.

Forgot to add I've repleced the one transfer switch, I didn't actually know theirs 2, think it'll be worth while changing the other too? Thanks.