Are you sure you took out ALL buildings (Except that indestructible one with the lorry parked out front - its part of the supply road)? Theres two very small churches that can be tough to see if they are still up among a lot of craters.
Did you kill the town ack? (its not shooting at you directly, its the 88mm "puffy ack", but it will kill the troops)
Depending on how far from town the field is, you need to kill field ack, too.
Its always possible a bailed out pilot sits directly in his own map room... hes got a .45 colt to execute the troops as they come in

. To be sure, strafe the map room before dropping troops.
Edit: I just watched the film. The thre large hangars close to the lake were still up. They look a lot like the indestructible building on the edge of town, but the are destroyable and need to be down.