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.