In reply to baileyswalk:
Was it recommended by the seller or the manufacturer? Was it designed to be an internal push fit or an external one. If it was in internal push fit but you are trying to put it on a smaller spigot as an external fit there will be dimension problems, the 'cone' will taper the wrong way. Also vice versa.
It is some years since I was in the flue/chimney industry, but many people suffered from wrong advice, often dangerous. If the seller or manufacturer recommended it, and/or sold it along with the stove, then they are liable to give you a refund. If you went in with a tape measure and bought it off your own bat you may have a problem.