Here's how I get the same problem...Inside the boiler there's a small tank of hot water. A pipe from the cold supply passes through this tank, and so the cold water is heated and comes out of your hot tap. There's also a temperature sensor to makes sure the hot water you're getting is hot enough. But the cold water passing through the hot water tank cools the water in the tank. The temperature sensor detects that the hot water isn't hot enough, and closes a valve slightly to reduce the flow, and so try to get the water up to the required temperature. How well it's able to do this will depend on the heat rating of your boiler.