Well first off, wherabouts are your bees located? If you're up in Montana where it's still snowing, or in PA where it's 90 degrees, it may change the advice given.
Second, what's your hive configuration? Entance at the top or bottom? Are they having to cross the empty comb as they come and go?
Third, if it's warm enough where your bees are for them to break the cluster and they aren't taking syrup, it just means they've got some natural sources that they prefer. If it's too cold to break the cluster, they may not be able to get to the syrup.