Pop off the top and check the big rubber O-ring. I’ve been very lucky in using a non-petroleum grease to revitalize the gaskets. Not sure were you live, but in the States you have a choice of Leslie’s Pool Lube or, at Home Depot or Lowe’s or any good hardware store or plumbing supply, a Plumber’s Grease that is Silicone or Teflon based. Do not use Vaseline or any other petroleum based grease…that will cause the rubber to swell and fail.
Just lube up the O-ring and reassemble. If that doesn’t work, it’s time for a new O-ring which can be had at any number or “parts” sites online.