My house is on the fifth floor. Is the main bleed valve blocked? Isn't the water supply pipe hot?

Put a little more until the hot water comes out. Do it again. Your situation may be that the water supply pipe is blocked or the valve is stuck, and you only release the water from the return pipe. If there is a filter on the water supply pipe, take it apart. And you have to ensure that the supply and return pressure in the pipeline is enough to form a loop.