Have you applied oil to your chest to improve the seal? Connect a single tube from pump to one dome and try with just one side.
Put the dome on a pan with water and pump to make sure it can draw the water up into the cup as a test that there isn't something wrong with the system.
Put the dome on a pan with water and pump to make sure it can draw the water up into the cup as a test that there isn't something wrong with the system.