My menstrual cycle seems to effect my Lexapro (escitalopram) making it now as effective. Why?

Complex system. If it is a recurring pattern, odds are it is a result of chemical changes throughout your cycle, such as changes in progesterone. Without knowing your full history it is possible that the current regimen is insufficient to address this occurrence. If symptoms are significantly impairing functioning it would be worthwhile discussing with your prescribing provider to consider options.