Test first, fix once... Remove the wheel..squeeze brake lever.. WATCH to see if both sides of the brake respond the same... if so, you have an ALIGNMENT issue with the Brake Caliper... if one side barely moves, you have a sticking brake problem.
you can solve an alignment issue by loosening the mount nut and then closing the brake on the recently reinstalled wheel.. then tighten the Nut CAREFULLY While Holding the Brake Lever Tightly, Watching to ensure the Caliper doesn't twist out of position as you tighten.... i hold the caliper as i tighten the mount nut.