Time of day, and even the season will make a difference, since the sun intensity changes. It is strongest at noon. You are right about the fact that cyanotypes don't do that well with subtle gradiations. That is why you usally see prints that have a strong "graphic" feel to them - hard sharp edged things with strong contrast. Dense and contrasty negatives do seem to work better. I think there may be some sort of "self masking" action during exposure but I can't remember for sure. Some people have built up UV light boxes in an attempt to make everything more predictable and controllable, but I never did. Like you say, there may just be too much variation from one sheet to another to make it worth my time. All I can say is, keep the prints that are good, and hide or dispose of the bad ones. One day every print will look great, next day you can't get one decent one. On the plus side, the darn things will last forever. It's one of the most durable and archival processes there is. The images will last until the paper itself starts crumbling into dust a few centuries from now.