Help! Positive wondfo OPk, negative clearblue?

This is my first

IUI

Cycle (took letrozole on cd3-7) and am currently waiting for my ovulation/positive opk to schedule my

IUI

.

My fertility center tells me to use Clearblue digital opk and to call when I get a smiley. Just to be sure not to miss my ovulation I’ve also done Wondfo OPKs to see the lines get darker.

Well today I’ve got a clear positive on Wondfo but the Clearblue is an empty circle. I tested with FMu and then 4hours later..

They won’t schedule my

IUI

based on Wondfo. Has this happened to anyone else, could Wondfo just be more sensitive and give me a positive sooner than the CB or do I have a bad batch of CB? So stressful...