gailmo,
I had the same thing happen about 2 weeks back: set out sugar syrup to open feed and the girls didn't touch it for two days, even added a little HBH, and finally pure honey around the edges of the container. Then, out of the blue, on the third day, I guess one of them found it and now they are wolfing the stuff down daily.
I, too, thought maybe they had another source, but the way they went after it once they found it, and since, I think it just took them a while to find it. Why, I don't know. The ants sure get there fast enough.
I am just putting out 4-5 lbs of sugar mixed in water a day for my 3 hives. It is all I can afford. The price of sugar keeps going up, up, up.