About to try first IUI, and no +ve ovulation result? HELP

I'm so frustrated! I've used ovulation predictor kits many many times and have never had a problem. I have regular cycles (things change slightly due to Letrozole now) and usually ovulate between days 13-19.

Of course the first month we are slated to try for an

IUI

... I never got a positive ovulation test!! I had super bad cramps on CD16 on the correct side that my U/S showed I had 2 enlarged follicles. Went for blood work BC I continued getting negatives ... And sure enough - I had already ovulated and couldn't do the

IUI

this month. I'm devastated!

Can I ask for tips on how ppl are testing? My RE says second void of the day, and before noon, is the best pee. Tips?