PulsaCoil A-Class running out of hot water

Hi, can anyone help me please? i have a pulsacoil A class that is two years old. For the last month or so i am having problems with the amount of hot water. I am allowed to take a 10 minute shower, but when someone else wants to use the shower, the water runs luke warm and then cold, this also runs cold in all the hot taps within the flat. I was led to believe that the a-class didnt use a tank of hot water, and instead heated it on demand and thus should give me a lot longer run of hot water. ive checked my top up tank, filled to the correct level, and ive tried pressing my boost button but cant tell if this is having any effect.
I cant be sure this is broken, but want to know whether i should call someone out or not, as i dont want to call someone and be told that its working perfectly. Can anyone save me?!
thank you so much
Sarah

What size is the unit?
You need to have it checked really
Mike