If your float not setup correctly, it is possible for the needle to be open and not close when float bowl is full. It is also possible for float twangs not allow enough fuel flow into bowl when bowl is pressed down by primer or when on bottom of dry bowl.
A propertly setup carb should not even need a primer button to refill float, it should be automatic when bowl is dry and needle is opened already to allow fuel flow. In my opinion, the primer works more by allowing displaced air to evacuate the float bowl faster by going past the primer button allowing faster fuel fill then just pressing the float down....a dry float bowl will already have the float in down position.
You can double check my theory by draining float bowl with drain screw (fuel petcock and lines free of fuel), open fuel petcock valve and see how much fuel comes out of the dry float bowl (remember, float is already on bottom and needle valve should be open for fuel to flow). Press primer button about 1/2 way which won't touch float or push it down further (remember, in dry bowl, float is sitting on bottom) but is enough to break airseal and watch the fuel flow rate...should be greater than before.