It really shouldn't be temperamental
I'd be checking the linkages to ensure that pushing the lever across fully rotates the locking dog pin
Assuming that pin rotates smoothly - the internals are generally robust and the sliding dog will lock.
It is recommended that if you are slipping any wheels to engage the centre diff lock - the centre diff does not love speed differentials and will wear quickly - a 2008 should at least have the moderately weak 2pin centre as opposed to the abysmally weak single pin centre?
S

