There is information missing but from what you have told us one possible cause may be that you have a leak somewhere in your system and the hot water is escaping before it can be heated up.
Look around for damp patches and strangely damp or humid rooms. You may not like what I'm about to say here but there is a chance that any such leak could be in a nasty place like under a floor.
Other ideas will come when you tell us how your cylinder is heated.
An immersion heater could have faulty stat or time control
If it is heated by a boiler then it could be an airlock in your primary circuit or perhaps a system fault such as a motorised valve not working properly. What is your boiler stat set to ? Is it working properly?
If you can tell us whether or not the flow pipe to the cylinder is getting hot this will be the biggest pointer as to the possible causes of your problem.
Have you had any other related work done or problems lately?
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