Gas not heating water for shower???

Hello,


Main bathroom shower barely has enough hot water for one short shower after gas boost has been on for an hour but water is still hot in kitchen / ensuite etc.

It is slightly better with immersion on for one hour but still results in a very short shower, very hot water for very short time, while water is hot elsewhere in the house.

The temperature is also hard to control and the shower has a boost unit for pressure.

Any ideas what’s happening here? The landlord said that they had the gas system serviced prior to our moving and admitted the shower wasn’t great but as it is it’s basically unusable.

Any ideas what might be going on here - the gas seems to work well everywhere else for water / heat.

Thanks