Usually no such thing as flat tire rules exist in Europe.

For international long haul, usually if you miss any flight, your ticket is voided if non-changable. With a bit of luck, the airline could reintroduce it for a later flight if talked to. I've found the North American airlines in general being rather generous on this. But given this ticket has more than one airline's metal (A3 and United) it might be trickier. I assume it's a United ticket? Then I would contact United and explain the situation. If it's changable (especially after departure) then you could just change it, otherwise try to be nice and explain the situation.

Worst case, be prepared to buy a new ticket..