Hey Cruiser,
Sometimes the neck of the valve can stretch if the plug is left in. Ie most newer kites have a deflate and an inflate, the inflate is left plugged and the deflate is always left out after use.
This inflate plug stretches the neck. The trick to solving it is to boil up some water and place the valve neck and body into the water for a few minutes and allow to cool with the plug "OUT". This should reset the "memory" of the plastic and it will seal well again. If this fails, the good old cable tie or insulation tape will help.
Good winds,