It sounds as if your expectations are not in alignment with your nanny's. You will need to come to new, agreeable terms with her or find a new nanny.
I had a very successful arrangement with a nanny in which I only paid her for the hours she worked. However, I guaranteed her a set (part-time) schedule per week. If she called in sick, she did not get paid. If I asked her to stay an extra 15min, I paid her for the extra time. She knew upfront that I would not use her during my husband's vacation time, and I gave her almost 2 months notice of when that time would occur, so she could arrange a side job if she wanted. We had discussed all these details upfront and both of us were satisfied. (We also gave her a huge Christmas bonus, as we loved her and wanted to give her a generous gift!)
Your nanny is dependent upon her income -- just as you and I are dependent on our own family's incomes. She has to find a way to pay her rent/mortgage, utilities, food, just like we do. After leaving us, our nanny took a part-time position with a family that offered her "about 20 hours a week" with some varying weeks. Then they ended-up cancelling 60% of that time with a day's notice. Our poor ex-nanny was left struggling to make ends meet and even pay her rent. This is a terrible situation for anyone.
Keep looking for a new nanny that will agree to your terms. Iron-out the salary and payment expectations during the negotiations so you don't run into this problem again. And Best of luck to you and your family.