28 day cycle, OVU CD18?

ESC

Hi all! First time posting... I’m turning 36, live in Hawaii, ttc 2 cycles now with my husband. Have tracked with OPK the last two months, my cycle is *exactly* 28 days, but I don’t get a positive OPK until CD16/17... Wondering if anyone else experiences this?? I’m worried there is not enough time in my luteal phase 🥺