It isn't quite so straightforward. Some people don't see mucus (which is there, just not copious) or could only see it once a day and if you aren't looking just them you will miss it. Not having noticeable mucus on its own does not necessarily mean no fertility. A lot of things can affect the amount (diet, supplements, dehydration, stress, etc.) without significantly affecting fertility.

I have no fertile quality mucus and I usually don't ovulate for another 5 days in a normal cycle. I've read TCYF and I haven't started charting yet. I plan to next cycle so... they say this monitor is 99 percent accurate, so can I assume I ovulated? Why no mucus? I usually do have some. According to TCYF, no mucus means no fertility, correct?