Make sure they are properly gapped,
DO NOT Pry on the center electrode to gap them!
DO NOT bang on the 'Ground' electrode to close up the gap!
Modern 'Resistor' plugs are VERY SENSITIVE!
Use a pair of NEEDLE NOSE pliers to bend the 'Ground' electrode instead of PRYING on the center electrode! That's the WORST thing you can do to a spark plug (Prying on the center electrode or 'Tapping' the gap closed!)

They have a 'Carbon Pile' resistor inside the insulator, and if you break that very fragile carbon, 1/6 of your horsepower and torque goes right out the window!

IF YOU DROP A PLUG ON THE GROUND, REPLACE IT!
I can't stress this enough!
This is probably the #1 reason for 'Loss Of Power' or 'Poor Economy' complaints after a 'Tune Up'!
People just don't realize how fragile the plugs are!

You don't need a ton of 'Never Seize' on the threads, but you DO need to use it (Copper if you can find it, but zinc is OK if you can't turn it up easily or Copper is too expensive for your project)