could be a number of reason, but first try turning up the temperature on the control at the boiler. you should find a dial, or sometimes two - one for hot water temp and the other for heating/radiators.
this assumes that you are running a combi system, ie mains fed and you don't have a HW cylinder. if you do have a cylinder then this too should have a thermostat. check it and turn that up too - don't go above 55 degrees C however.
if it is a combi then you can also try turning down the tap a little, ie reduce the flow rate. this will give the boiler more time to heat the water.
if these things fail then it indicates a fault. you need to get your landlord to sort it out. threaten to withhold rent until the work is done! this normally gets a hastier response.
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