Hey Mountain

I don't know which ignition system you have, but I have an idea.
If the plug is the correct one, and the battery is in good shape, then you pretty much have to look at the ignition system.
The ignition system may be defective, so I would look at getting an replacement.

Oh, by the way did you properly gap the plug? The gap should be from 18 to 25 thousand... your manual would tell you exactly.

Good luck

Henry