Had an interesting situation today. Playing about putting new tubes and starters in four fittings, found one pair of tubes where when new tubes and starters fitted the tubes didn't come on. When I put the old starters back in with the new tubes they came on. All tubes and starters were compatible and from the same manufacturer. What could cause this? My guess is that a used starter is more bendy (technical term) through use and it's a sign that the ballast is on its last legs.