Probably got some gunk built up in those lines. I recommend a purge product, their are several out their try not to get to strong of a formula. A couple are recommended here. I like Swirl Away seems strong but not to strong. Could be because it's available at our local plumbing wholesaler. Be prepared to rinse out the tub a bunch. Make sure to take your filters out. Follow the directions. I use a bit more than is recommended.