Says it all...
Combi installations don't usually suffer from airlocks, as there's mains pressure behind every tap. It may be that the stopcock isn't fully open and is restricting the flow (presuming you've got mains feed to the taps all through the house, as with the usual combi installation) since you turned it off to change the tap washer? The initial pressure will come back up to what's in the mains, albeit slowly, every time you turn the tap off (through the restriction), and drop again when it's opened, leaving you with the disappointing dribble that'll pass through the restriction. If you're really unlucky, the stopcock may have broken inside with the "puck" on the end of the spindle jammed against the seating (more likely if it's been fitted the wrong way round with respect to the flow, which normally will push it off the seat) - if so you will need to either get the water company out to turn the street c*ck off while you replace it, or get/make a tool to do it yerself...
I personally *hate* combi boilers, never been to a house with one where you can rely on not being alternately scalded and frozen in the shower!
Hope this helps, Dave H. (The engineer formerly known as Homeless)