:welcome:
Yes it is a common problem. if you'd done a search you would have found answers to this. when it hasn't be used for a long period the linkages can seize up. Give everything a squirt of oil or penetrating fluid on the linkages see if this helps. If not release the linkages from the transferbox nob and use a 13mm socket and t bar to turn the nut on top of the difflock spigot. This should put it in and out difflock, do it quite a few times till ya happy it goes in and out and then lubricate everything up and put linkages back on.
This may be tad confoosing but do a search and you'll prob find a better way of explaining it!