i have a problem with brakes occasionally sticking on front wheels(disc brakes)

just every now and again the brakes stick on one or other side front wheel.does not seem to occur for any particular reason. i took off calipers etc. and pushed piston back in and out again. when out i sprayed wd-40 down inside the rubber boot around the piston. jeep only purchased 9 months ago and looks like new brake pads fitted before i bought it