this is something we all come across many times, first thing to check is the flue to see if it is clear, the spillage test is an indication that it is clear and exiting outside as a 5m3 pellet draws up it, however when you turn the appliance on, you have 10m3 of air for every 1m3 of gas so the volume of "stuff" going up the flue is more than double and a partially blocked flue will kick the POC's back down, which is why i urge everyone to ensure they know how to do a spillage test properly and ensure they do it every time,
once you have confirmed the flue is clear you can then look at the bottm end of the install, type of flue suitable for the fire (bit late now but its in now) check catchment and flue opening size,