Ive been suffering from same issue and to help identify issue please try this.
With rear wheels on ground watch the lever the handbrake cable fits into at caliper as someone operates handbrake inside car.
There are lever travel stop pins on caliper, check travel of lever mechanism is equal both sides and that handbrake cable returns fully both sides, if caliper fault then likely stuck internals, best to exchange unit.
Otherwise maybe cable due for replacement.
Hope this helps
