it may seem a bit odd, but if you try to turn the stopcock the other way ( further open ) before
trying to turn it off you may find this will help.
failing that, if the stopcock is metal rather than the newer plastic types, try applying a little heat to the handle carefully with a blow torch allowing it to cool, re-heat again and allow to cool. repeat several times and when it has finally cooled you may find it will close . do not heat it excessively or you will damage the inside of the stopcock.