Did you slip off the coil pack connectors and check the pins?
Also if you are not using a torque wrench then remember that there is a crush washer on each plug.
With the car ticking over, pull the coil pack connectors off one at a time in turn to establish which cyl.is causing the issue.
The plugs come preset, however it wont be the first time that someone has accidently knocked the electrode while fitting thus closing the gap.
Finally, might be a faulty new plug, however I have literally sold thousands without issue.
Alex