There are many reasons why a fire will fail a spillage test. The fire itself may be faulty but that's another matter. If it had an obstruction other than a birds nest then the integrity of the flue may be suspect. Can you actually see inside the chimney and look up? Look for any obvious signs of wear. Light a match at the spigot opening and see if the smoke is drawn in. Ensure that the chimney is sealed as much as possible, use a smoke pellet to check for continuity of the chimney along with any leaks (look in roof space). This weather does not help with a spillage test, so the fire should be tested at 5 minutes and then again after a further 10 minutes. Does the room have ventilation? Are there any extractor/circulating fans? I would really recommend getting a local engineer out to investigate further.
Wal, April 2011