there are 2x slides (pins that the caliper moves in and out on) that the caliper moves in and out on, they are usually bolted or welded to the rear caliper mount.
if you ever high pressure clean your bike, it may have got water/mud etc in there past the rubber seals.
this can cause rust on the slides, and can cause the caliper become sticky or to stay in one spot.
if this happens you wont have very good rear brakes, sometimes none at all.
if you take the rear axle half way out and drop the caliper and mounting plate out, you may find the problem.
try and slide the caliper away from the plate, if it is sticky or wont move at all,
then you need to pull it off completely, carefully pry it off without damaging the caliper.
clean up the 2 pins that the caliPer slide on, and use some rubber grease on the slides and inside the caliper slides
here's some from SCA-
http://www.supercheapauto.com.au/on...Disc-Brake-Lube-6g-BBL1.aspx?pid=323995#Cross