I need some insight and expertise.

I'm currently on CD 12 after taking Femara on cycle days 2-6. Yesterday I had my mid-cycle scan and had two mature follicles measuring 18mm and 20mm. I used OPKs yesterday, this morning and again about 10 minutes ago and they were all super negative, like barely a hint of a 2nd line.
The same thing happened to me last month and I ended up having to trigger when my follicle was at 27mm, which may have been too large. I didn't get the trigger this month since I was really wanting to ovulate on my own, but after a negative OPK last night I ordered it and will use it tomorrow morning if I don't see a + before then.

Does anyone have any idea why I wouldn't have a natural LH surge? I've always ovulated fine during natural cycles.