Are both sentences right?
They could have won the game, but they were too tired
They could have won the game, but they had been too tired?
Could the first one refer to the beginning of the game, and the second to the whole game?
The first one suggests they were tired when the game started and for the duration.
The second one suggests they were tired at one point in time before the game. This doesn't really make sense in this instance. However, you might say, "They could have won the game, but they had been drinking the night before."