A new boiler and the shower has no hot water. Cold water flow is fast. Any ideas on how to fix the problem?

I have a new Viesmann 50-W boiler installed two days ago by an engineer from a company. There is no hot water when using the shower (upstairs) because the boiler does not turn on to heat the water. The hot water taps (up and down) do work but at a slower rate compared to the cold water. The cold water flow from the taps is fast and the house water pressure is between 2 to 3 bars of pressure.

A guy came out and said that there might be an external water tap decreasing the flow but the cold water is fast. Also I do not have an external tap. He then talked about a damaged part because the boiler is trying to heat the water too quickly and shutting off. But why would you get hot water through the taps? I think it is the hot water flow and it is not triggering the boiler. The shower is very close to the boiler. My understanding is hot showers can draw less hot water thus less water flow. The shower worked before.

Does anyone know why this is?

A company surveyed the house and a engineer installed the boiler. I also have 10 years guarantee. I feel that they should not of recommended this boiler if it is not compatible. I felt that they were trying to brush me off. They installed the boiler but does not work in the bathroom.

I would like a second opinion so I am asking here too. It is hard to explain but feel free to ask questions.

Thank you.