The majority of my colonies will take down 2.5-3L of thymolated syrup in about 4 hours at present and that is alongside varroa treatment with ApiLife Var. A few colonies wouldn't take that amount down in the course of three or four days. The slower feeders are the ones who have maintained their own stores in the BB over the summer. I do note that this past week all my colonies have been quicker to take down feed.
When you first feed bees with any sort of syrup, it can be useful to tricke some of the syrup through the feed hole onto the tops of the frames so that the bees know the syrup is there - with rapid feeders I make sure to trickle the syrup down the neck of thefeeder that the bees climb to access the food. They will follow the food trail to its source.
As others have noted, disease could also be a reason for them not eating the syrup...