Samsung washing machine does not retain water?

Model no:WA10V5 (top loader), Brand: Samsung Scenario: At the very start of washing the water inlet valve opens but the water immediately starts draining from the drain pipe and is not retained in the tub. By manually blocking the drain pipe, when the water fills up, the machine starts normal wash. The small motor at the drain pipe, when checked independently is OK and spins when supplied with power. what could the fault? How to locate and repair it? Thanks.