My 2004 V70R tailgate will not lock. All the doors lock fine with the FOB or the lock button, and when I open the 'locked' tail gate, the alarm goes off. That's telling me the car thinks the tailgate is lock.
Maybe a fuse issue?

 

this may or may not be a lot of help, but I have a 2000 that did/does the same thing. it is electrical in nature, and I believe the issue to likely be in the wire loom where it jumps from body to tailgate---- rear wiper and power lock no longer work, and cargo light comes on as door is opening but goes off as it is opened fully. tag lights still work, however? alarm sounds if tailgate is opened while rest of car is locked. havent dealt with it since you can still lock/unlock a 2000 manually from inside if necessary--- not entirely sure why they deleted this feature on the 01-07 years