It has to draw 100%, If it doesn't and you use it, the Gas Safe engineer that commisioned it will be liable for any deaths/ injuries from the appliance, Gas fires all produce CO due to how they burn, so it must pass the test 100%,

I would never allow anybody to use it on any of my installations unless it passed.

How does the flue terminate ? (ridge vent or normal chimney?)

Have you swept the chimney before installation ?