those giant springs get weak.
no one seems to want to replace them but they stretch like hell.makes a big difference with a new one.Sometimes you can find an industrial spring dist and match it up close or get a better one.Keeps you from getting raped by Vermeer.
another bit of advice.take the rubber mat off the top of the roller area and clean all that crap out of the top of the rollers.shop vac works good.all that grit eats the seals and eventually the bearings.anyone who has redone the roller bearings knows what a lousy,miserable job it is. Most times you don't know they have failed before they seize and the shaft of the wheel motor snaps.