I had the same thing happen to me earlier this month. Tried to use BA miles on TAM from Sao Paulo to SLZ, but BA did not even have SLZ in their system. Then I tried GRU to IGU (another itinerary), TAM had seats but BA did not have any seats. Calling did not seem to help.
In the end had to buy the tickets, especially the CGH to SLZ turned to be fairly expensive due to the time of the day I intended to arrive.