it can definitely happen. FF just goes on your temperature and cm. Some people may never get cm, or may get it at the wrong time of their cycle...and others may have very wacky temperatures like I did on the cycle I conceived!

I definitely ovulated on say 14 (i know this from blood tests) but FF put my coverline and ovulation day at cd18! All because my temps did not go up until then.

But even if I put a postivie OPK on there, it still wont change the coverline! So dont despair, hang in there and fingers crossed!