Works one cycle and not the next

A quick background:

- unexplained infertility

- 5mg letrozole days 3-7

- currently in 4th cycle of using it

- cycle 2 monitoring I had 3 follicles

- cycle 4 monitoring I had 1 follicle (increasing to 7.5mg for next cycle).

I was completely blindsided when I went in for my ultrasound yesterday prior to starting iui and only had 1 follicle especially since I thought I was responding super well to it based off my 2nd cycles results. I should have asked more questions but was shocked and then was told we could cancel our iui (which we ended up doing in the end). Has anyone had this happen? Do you know why it happened? Now I’m concerned next cycle I’m going to have 5 follicles with the increased dosage and will be forced to cancel my iui. Hopefully that one egg from this cycle is a lucky one and our non iui efforts will work out for us this time!