Right im after some advice please guys its not ford related but i know theres some switched on people here

basically on my k11 micra its been smelling of burning from the N/S front wheel and using more fuel than usual.
i decided to remove the wheel and found still a fair bit left on the pads but the pads were being held tightly on the disc and this must of been the cause of the burning smell.
i took the caliper off with great difficulty as it was being held so tightly on the disc and tried everything to free the piston up eventually i drained some brake fluid and got the piston back, it just meant the brakes needed bleeding as the pedal was hitting the floor.
i bled the N/S front today and then went on to bleed the O/S front and that one is now stuck on with non existant pads!!
obviously im going to get some new pads in there but anyone have any advice on how to get the piston back and also as to why its stuck??

many thanks and sorry for the long post!!