What’s reasons for not ending up pregnant?

If you are trying to conceive and tracking ovulation and know you have had sex all around your ovulation days and other weeks too just to increase your chances and feeling ovulation happen with ovulation discharge and not conceiving what are the reasons to why there was no baby made? I’m just curious as I wonder if I may be infertile or if it’s just common to happen
What’s reasons for not ending up pregnant?
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