Very little hot water from my immersion water tank?

I live in a house that only has electric, the radiators are not run from the immersion tank but the water is, we have gone from having enough hot water in the morning for 3 showers to just one then the water is cold for hours! the timer is set to be on 24hrs a day!!! WHY????
It appears that the tank is only holding a small amount?
Any suggestions??
Regards