Firstly you should inspect whether there has foreign objects on the duckbill rubber check valve, foreign objects may cause lodged between the lips of the duckbill rubber check valve and could prevent the valve from checking.
Then verify that there is clearance between the bottom of the bill and the ground and that no debris is trapped there.
If the duckbill rubber check valve has blisters, delamination or deformation inside, it indicates that the medium or over high temperature are causing damage to the duckbill rubber check valve, you should replace the valve ASAP.
Also inspect the duckbill rubber check valve for any cuts, checking and fissures. Repairs can be made on site with a repair compound for small cuts. But if significant fissures are noticed where fabric is exposed and torn, the check valve must be replaced.