Fixed! Used a flashlight to check both the plug and the socket for contaminants. Wiped them out with a Q-Tip. Immersed a fresh Q-Tip in Super Lube (a synthetic lubricant containing PTFE) and coated the working parts of the plug and socket with a thin coat, then plugged in and charged the car...
I will check the o-ring and also try the other suggestions and keep you all posted! I appreciate the many responses! Love the Leaf! It replaced my 2004 Chevrolet Avalanche.
I charge about once a week from a 120VAC outlet in my garage. Over the year I have had my 2018 Leaf SL it has gotten harder and harder to insert or remove the plug. Any ideas?