Registry Question

I have a question for anyone that works registry. Our hospital has a 3 tier/level system for registry requiring different amount of hours, weekend shifts, and holidays occuring to pay. I have worked the top level for 3 years. I am required to work 64 hours including 3-8 hour weekend shifts and one summer and one winter holiday. I have always managed to get these without a problem. We use sign up sheets and you just sign up for what is open. Well for the last 3 months there haven't been enough weekend shifts to sign up for...usually I work 2 12 hour shifts and that is okay by them. Now they have 4 agency contracts due to the short staffing issue and I have 12 hours this month and they are threatening to drop my level down which would mean less pay. My contract states I must give AVAILABLITY for the required hours....they are now saying I must work what they have open even if it's midnights, days or whatever. So my question is how do you get your hours and how do they get assigned? I am almost ready to drop down to the lowest hours/pay as to not have to worry about any weekend or holidays since I am so mad.....I love my job, but they are making the scheduling harder and harder.....We have a schedule nazi and I believe they are trying to strong arm us into working when they want us to. isn't the point of registry to allow yourself more flexiblity?