Ariston Eurocombi Boiler 23MFFI ?

We have had the above boiler for 3 years and can only get hot water if we turn the water and heat temperaures on full. This has happened since it was first installed. We always thought that the pipes in our house were too big and the water didn't get time to heat up but when a plumber came recently he said that it had never been altered from its factory settings but when he tried to fix it it didn't make a difference and he couldn't find the answer. Could it be that the boiler is not big enough for our house (3 bed semi) ? Would love to fix this problem.