When the pump is working on one side only, that means there is something wrong with the connections or parts on the non-working side. The pump doesn’t have the ability to choose sides, it simply suctions. Please check the following:1. Ensure the T-Connector selector is in the central selection.2. Ensure the silicone diaphragm is seated correctly.3. Ensure the white cap is locked firmly into place (you will hear and feel a subtle click).4. If you have the massage cushion in place; make sure it’s pushed in and seated well inside and all around the lip of the flange.5. Make sure the duckbill has no tears and that it is tightly seated.Please go through this list to remediate the issue you are experiencing. If the problem persists please email us to set up a call with one of our Lactation Consultants who can assist you in the troubleshooting process.