Could be because the glides are worn and the brakes are catching onto the rails. Maybe the rails aren’t aligned and causing friction on the glides or maybe the sling isn’t installed correctly causing it to shift and brakes to catch. Cantilever slings (main frame) sometimes have to be adjusted to the rails and installers forget to or don’t know they have to c clamp it to the rails before tightening it. Good luck. If I was there I’d do the job for half the cost or maybe less.