BBT Coverline Help!

Jenny • New Mommy to Jolie, Veterinarian and Husker Football Fanatic❤️
Ok so I am posting my bbt chart for some input from my glow ladies.  I am getting +opk's and usually a spike in temp to at least 98.5 post ovulation​. This month however, even though the opk's are positive my cover line temp only went to 97.9.  I am worried maybe I did not ovulate?  Can the cover line change monthly?