Pathetic service mc Donald’s

Everytime we stop for ice cream at the drive thru after waiting for up to 40 minutes sometimes, we get to the window only to be told they dont have ice cream. Why don’t they put a big sign before one joins the queue that says WE DONT HAVE ICE CREAM - they are so pathetic to make customers wait and then wait to get out of the queue, very poor service and very inconsidered