That happend to me after I disconnected my handbrake cable and raised the handbrake vertical to get the centre console out (Disco TD5) for a bias plate replacement. When I put the handbrake back together the button was sticking. I tried oil etc but it was no use.
What's really frustrating about this is that it is such a simple mechanism, but on the TD5 at least, the part of the handle containing the button appears to be sealed.
The only way I could get the button out was to hit the handle end with something or crack the handbrake down against the stop.
I orderd a handbrake off ebay, 15 quid inc delivery, fitted it this afternoon and its as good as new. When I raised the handbrake to get the centre console out I was careful this time to keep the ratchet engaged.
Cheers
Andy