baxi 105 combi cold showers?

I don't know if this is a problem with my boiler or somewhere else. Our boiler works okay with the central heating and everywhere else in the house i get hot water when I turn on the taps. But when we try to use our shower there doesn't seem to be enough hot water running through the system to fire up the boiler. to get hot water through to the shower i have to turn a hot tap on at a sink. when i do this, hot water also comes through to the shower although there's not a lot of water coming through. when the shower just has cold water coming through the pressure from the shower head is great. when i turn it to hot water, it turns into a dribble.
when we first turn the shower on, it also makes a funny noise as if air is moving through the system.