Crack open the bleeder screw and see if some fluid shoots out and it loosens
up or not!  If it does, I would look at replacing the brake hoses(Both
sides!)  They may look ok on the outside, but be bad on the inside.  Know
why brake fluid turns black over time?  It's from the deteriorating hoses.
In time they can internally collapse and block the fluid from releasing.
You also need to lube where the caliper is sliding at with good High temp
Caliper grease.  Or you just did something else wrong.  Was the same side
not releasing before you took the brakes apart?