Here are some points to check:
Check your alignment: are all parts fit snugly together? Is the diaphragm pressed firmly into the back of the hub? Did you forget the duckbill valve? Are the holes at the end of the flange lined up top and bottom as they should be?
Make sure all of your parts are dry
Replace your parts: the silicone duckbill valve and diaphragm are parts that need to be replaced regularly. Depending on how often you pump and how you clean your pump parts, the duckbill valve may need to be replaced monthly or every few months.
If you have tried all of this and you still cannot feel suction, contact our customer support team