Just sharing my actual experiences. We asked our rav on both of these occasions.
1) I sat down on the couch on Friday about 40 minutes before candle lighting and woke up only after shkiah. I had not intended to sleep and at the time I had a 2 month old baby with very bad colic and was lacking sleep. Our rav said that it was not in my control and that I did not need to add a light.
2) I had a very busy erev Shabbos and just one more thing to do before lighting. I did the one more thing and, being very pleased with myself at how organized I was (the house was done, the food and table all set, the kids were all showered AND dressed!), went to get dressed. I finished getting ready and came out of my bedroom and saw the candles weren’t lit and that it was after shkiah. I had literally just forgotten. I was horrified with myself as I have been lighting in my home for 12 years, but there was nothing to do. Our rav said that it had not been prominent enough in my mind and I was negligent for having failed to light and we added one candle. I wish I could say that I never run to candle lighting anymore, but I definitely always light!
Hope these examples help.