Most probably you have some sand or grit in the pump valve stopping it closing.
Carefully remove the screws at the top, outside ones and under you will see small round disks about the size of a 2 cent pieces. Remove any sand grit etc. and carefully put it back together . Should now work. Take your time and don't force anything, there are rubber seals around the edge and will come apart with a small amount of effort.
Ta
John