When one piston is only traveling, it'll cause that symptom. If it is stuck & not as a result from a empty pad lever pull; What tends to help get both moving again is pushing both pistons back at the same time, then as you maintain pressure on both pistons, pull the lever. Repeat until both pistons are moving is sync. If that fails, reverse bleeding would be the next step. If the system never lost fluid & has not been breeched with air, I try to go with the noninvasive methods first.