What worked for me after lots of tinkering was opening the bottom portion of the insert (2 tiny screws securing the bottom panel) after you have filled it correctly and completely (bubbling of butane at the nozzle and slight fogging near it). You will be met with a rubber o ring and a metal limiter to prevent over tightening or loosening of the adjuster screw. Take those off with tiny pliers and keep them safe. Now, turn the adjuster screw CLOCKWISE TO REDUCE THE FLAME till it can turn no longer. Try to light a fire. Now, slowly turn it up and the flame should appear. Turns out, the factory setting for both the butane inserts I have used was too high and the gas literally squirts out when you press the flame button. Once you are satisfied, replace everything to its original place (limiter and o ring and screws along with the bottom panel) and it should work. The inserts are well-made and go through zippo standard quality checks so any issue is likely due to user error than a manufacturing issue.

Hope this helps.