Cervical mucus not corresponding with BBT rise

Jamie

So my ovulation day according to my BBT is religiously on day 12 (temp rise on CD 13 and 14), however I find that on CD 20 I always have the most "fertile" appearing cervical mucus (raw egg white in appearance, very stretchy) in my whole cycle (27 - 29 days).

I'm confused as my CM is not corresponding with my BBT's suggested ovulation period.

Thoughts/advice?