You probably did not win. To win, you need sufficient mating material. If you are down to say, just a king, and you opponent's flag falls, it is only a draw. To get a win, you need to have sufficient mating material. A pawn is sufficient as a pawn can become a queen or room and Rook plus king is mating material. You opponent can have 3 queens, 2 rooks, and 4 pawns. Does not matter. If you have sufficient mating material, no matter how wretched the position, you get a win if he flags, but if you don't, it is only a draw.
Tough luck - it's the rules - no sympathy!
I often won the game on time, but the chess App gives me a draw, it makes me angry.