What happens if you go back to the smaller sized shields? Sometimes the bigger ones don't work quite as well (although they are more comfy).
Is sucking happening? And if you have recently hand expressed then there may not be much more left to get out.

Are you sure that you know how to adjust the strength of the suck on the pump - someone may have turned it down to super-woosy when you weren't looking.
And if it is a pump that is supposed to change from little suck to bigger suck after a minute then are you sure that it is changing correctly?