About 5 years ago, I started getting bad batch of Nova plugs. Pretty much you will blow a plug every run on the same batch. Switch to a different batch and things comes back to normal.
Moved to the Odonell a few years ago and never look back. They works like a plug should. If you run too lean they will still blow and they last much longer. Have not found a bad batch of plugs yet. Then I have not tried any from this year.
The best thing about the Ondonell is when they blow, they don't drop the coil into the piston and burn a hole on it!