You gambled, you lost. If middle seat is allocated, it probably means the flight is running quite full, even though not all seats allocated yet. Often seats are blocked from selection, to accommodate parties travelling together. FWIW, if you're going to play this game, best to do it towards the rear of the plane than the front. Forward middles will go well before rear middles. Some people are so obsessive about being towards the front of the plane that forward middles may go even before rear aisles or windows.