From the description you have given I would suspect that you have a restriction within the heat exchanger unit at your heater which is blocking the flow coming out of it to your taps etc.
It could just be that the unit is getting full of grit and suchlike – call your serviceman to come check it out for any blockages.
Hope this helps.
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians