Yep, it's usually dirt in the webbing. If it's bad enough you can remove the complete belt/reel from the car, pull it all out of the reel and clamp it with a bulldog clip so it doesn't retract. Then wash/scrub the webbing in wool wash and warm water using a nail brush. Keep the reel out of the water obviously. Wash the top pivot plastic thing too. Let it dry and it'll work like new again.