Mathmatically speaking, I can't give you an actual percentage but it can happen. Have you taken antibiotics - even for a UTI?
My doctor told me that if you take the pill, every day, but are off by more than an hour or two, that it will render some pills ineffective. Nowadays they are so sensitive! (that's when I switched to an injectable bcp - 1x a month and no more thought)
Keeping my fingers crossed for you. Keep us posted.