Yes, I think it's hard for FF to predict ovulation when lots of temps are missing. You also need three days of sustained temp rises for cross-hairs to appear showing your ovulation day.
I notice you had watery CM on days 17 and 18. Do you use ovulation predictor tests/kits, get ovulation pain or check your cervix position? If any of these other signs point strongly to ovulation, it's possible that it occurred between days 18 - 20. It's hard to know for sure though, that's just a guess.
Hopefully other charting nuts like me will post here too.
Make sure you take your temp everyday, and also around the same time before doing anything else. This will give you the most accurate temps and charts.
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