Lightning visible, thunder heard, but nothing on map

[Philip.Gladstone]

I've noticed that many times when a thunderstorm moves through, I will see the lightning, hear the thunder (upto 15 seconds later -- i.e. lightning was with 3 miles), but nothing shows up on the map. 10-20 miles away there can be strokes plotted.... so there is a sufficient density of stations around me.

I read elsewhere that cloud-cloud strokes are difficult to detect, but is that really the explanation here?

I'd love to be able to watch the map and the expanding sound front and have it be right!

Philip