1. First try cleaning or replacing the valve O-ring and while tightening the nut on the band clamp you must tap on the clamp as you tighten the nut starting on the opposite side from the nut. This will allow the clamp to draw up evenly. Sometimes the O-ring will swell larger than the groove and will not fit. You must replace a swollen O-ring.
  2. If it is still leaking, then the base may be cracked at the flange for the clamp. This requires a pool professional to replace. Note:  A cracked lid must be replaced as soon as possible because if it fails all the way it can come off and hurt someone near it or the pool could be partly drained and damage the pool equipment, the deck, or the interior finish and landscaping.