I wouldn't totally forget about it, I would make a note of it in the comments section. FF bases your CH's on a number of factors (especially depending on which setting you have it on) and can be quite fickle sometimes. I know it's given me CH's when I haven't ovulated, and other times FF has given me solid CH's and then taken them away based either on temp or CM. It's a real let down when that happens too.
Some people can have fertile CM after O, but FF doesn't like that. If you keep charting you may notice a pattern to your CM that gives you even more insight into your body's particular patterns, despite what FF thinks.