Small follicles on letrozole

I went in to my RE today for my first monitoring ultrasound as part of my first IUI cycle. The hope was that my follicles would be large enough to trigger today and do the IUI tomorrow. This is my first cycle on letrozole, and my first IUI. Before this cycle, I’ve always had very regular periods and ovulation. For some reason, the letrozole seems to be delaying my ovulation as my follicles are smaller this month than they normally are at this time. My RE had me do additional bloodwork to see what my levels are at to determine where to go from here. She is saying I might need another week and to put me on more letrozole. For reference, I did 5mg of letrozole from CD 3-7. Has anyone else experienced this? Is this a bad sign for my IUI cycle? It’s making me feel stressed and already like it’s not going to work. Please help!