coudl be one of many things.
could be the locking mech not working right

bit of oil/wd40 and a few workings might sort that out

or it could be electrical....  a wire off somewhere ... or worse a break in
the wire somewhere else.......

try oiling the lock itself.... thats a start...

My boot on the xantia is a bit awkward soetimes..   sometimes it'll lock
otehr times it wont.....

Also try locking it manually.. then trying to open it using the
remote.......you may be abel to hear something happening.  if not it could
be electrical...

good luck.. it wont be a major job ui'm sure

Rob