I am using Medela MiniElectric Plus( double pumps) at office and Avent Manual pump at home.
When using Medela, U need to be patient. Your milk may come out a bit and then appear to stop. Don't take away the pumps or move them as it takes time for increasing the pressure inside. NOrmally, it takes less than 10 mins for me to obtain the reflex (反射噴奶反應). U may wait longer to obtain the reflex for the 2nd time.
For Avent, the pumping action simulates baby's sucking. So, it takes less time to achieve the reflex and more comfortable because Medela pumps tend to suck out the milk by vacuum mechanism.