The owner of the Northwest's tallest building, the 76-story Columbia Center, missed a mortgage payment this month, providing fresh evidence of the troubles facing downtown Seattle office landlords. Boston-based Beacon Capital Partners failed to make a scheduled payment of $1.65m on a $380m loan it took out when it bought the tower three years ago, according to a recent report by Wells Fargo Bank, which administers the debt. The loan faces "imminent default due to cash flow issues,"says a note in the report. A spokesman for Beacon, the Seattle area's largest office landlord, declined comment.