Delayed Canceled Flight by British Airways

It is now two times that BA canceled my Flight from London Heathrow to Luxemburg.
My two Flight from the States was arriving at Heathrow Airport at 06:20 and my Flight to Luxemburg was at 07:55. Every time he says to me at the Fast Trach Line that the gate was closed at 07:15 Why I don’t understand that. that is not a good Reference for British Airways.