Brakes not always releasing

First of all, is there any chance the brake fluid got contaminated with a petroleum product like engine oil, transmission fluid, power steering fluid, or axle grease? If that is not a suspect, when the problem occurs, observe if the brake pedal is higher and harder than normal. If it is, there is a restriction in one of the lines, and that is likely the cause of the failure of the fluid pressure to release. Stop on a small incline, shift to neutral", place a block about a foot downhill of one of the tires, then crawl underneath and open the bleeder screw for the affected wheel. If that lets the brake release, that rubber flex hose is the best suspect. You can also loosen the lines at the master cylinder. If that lets the brakes release, the fluid is likely contaminated and the rubber lip seals have grown past the fluid return ports. That is a real serious and expensive repair.