There were dozens of witnesses who “noticed”.
They screamed, yelled, waved their arms at police and secret service, pointed to his exact location for at least a couple of minutes.
Those Trump supporters were close to the shooter and were yelling to the SS and police that “There’s a guy on the roof with a gun” The shooter was bound to hear them because they were close to him. The shooter probably rushed his shots and was not as accurate as he could have been.