Last month positive this month nothing

Jennifer
Last month, my Dr gave me Letrozole which is like clomid. I was so close to getting a positive result. So my Dr gave me a higher does for this month. The past 4 days I have been testing, I have not even gotten a line on the test side. Last month, it was almost as dark as the control side. This month, a line does not even show up. Why? Please help?!?