Mostly Champion Spark Plugs, resistor type gave problems
About 10~12 years ago, I started having miss fire/skipping issues with Briggs powered mowers and Champion plugs.
 Hard starting also. meaning several pulls . Typically my mowers start on the first or second pull after dry winter storage.
Always one pull afterwards for the summer. Shut off the fuel valve and let the mower run out the carburetor and quit after each use.  Open valve as you get the mower out and by the time you get ready to go, the carb will be filled.
 Air filters clean. Oil level good and clean. Fuel filters (yes I run inline filters and a cut-off valve in my push mowers) fresh non ethanol fuel. no chaffed wiring, loose controls or grounding clips etc..
 Fuel jugs are kept indoors and spouts are usually covered. Never left out in the weather. No trash or water in them.
 I'm a little picky that way.
 
A visual inspection of mower revealed nothing. Pulled spark plug and held to engine block and normal rope pull got a weak, intermittent spark.
 Switching to the clip on test plug and saw spark looked fine.
 Checked gap on all plugs, new and used.
 Grabbed a new resistor plug and did the rope pull and looked at spark. Spark looked fine with a new plug.
 
 Decided to grab the multi-meter and check resistance of old and new plugs.
I can't recall the specific numbers but plug from mower showed resistance of more than double of a new plug.
 Same type and possibly came from same batch, as they were all bought together.
 Bought some new ones and had exact same problems.
 Have had a few NGK resistor type fail to high resistance also. Hard starting was usually the first sign.
 I had to go round and round with the parts counter guy to get a Non-Resister spark plug. But worth the aggravation.
 
Running non-resistor plugs ever since and have had no more issues.
 All of the resistor plugs got tossed in the trash after a couple seasons of no issues with non-resistor plugs.