Predators, like a cat, wild or domesticated. That is usually one of the biggest reasons, although, your birds came back, so it could very well be old food. Birds are smart and won't eat moldy or old food, much like us. :) Unless you are a ravenous teenager.
I bought new food that they don't LIKE. I had to change it out with another feeder and put straight sunflower seeds in it. Of course, this really attracts them. Especially the cardinals and bluejays. Well, all of them seem to like it. Your cardinals don't migrate. We always had them all winter long, but, if there is enough out in the woods for them to forage, they won't hit the feeders. I have four feeders and countless birdhouses. They are pretty fun to have around.
PS.. Every once in a while, especially in the spring when it rains a lot, I will clean the feeders. Nobody likes eating from a dirty dish. :)
Yes, well after a few days of testing, it appears that it was definitely a case of "food gone bad". After moving the feeders, they were all over the other feeder in that same location previously ignored, and then started hitting the other one with almost as much gusto as the other one once I cleaned out the old food and replaced it in the other location.
Our cats are very spoiled indoor only toy predators, although at least a couple would likely go for mice if located.
I'm not much of one for cleaning, but suppose I will have at it now and again. The weather here in Vermont is a LOT more wet, humid and mold conducive than dry, dry Colorado was.
Oh, yes! New England in general is. I now look for easy-to-clean ones when I am buying them.