For separate Hot and Cold Taps - Have you tried connecting a short length of hose pipe from your cold tap to your hot tap.You may need to use hose clips ( jubillee clips ) to avoid soaking the place. For a mixertap - try and block up the end of the tap with a cloth or rubber glove and then turn on cold tap.Both of these procedures should unblock an airlock in the system if it is an airlock, an airlock may have formed if you drained your storage cistern,or if you had any work done to the system recently.I had a similar problem recently and this sorted out the problem.however I'm living in Ireland and am only familiar with the british/Irish system. Let me know how you get on.
Methodical Dave
February 2009