Sam,
A few points.
The colony may still be eating their honey/syrup stores in preference to fondant.
Are you sure that the feed hole is directly above the spot where your colony is clustering? If the colony is at all weak and the feed hole is not directly above their cluster, they may be very reluctant to cross frames and then go up above the crown board to feed (they would be having to 'endure' cooler air). If you have an open mesh floor with an inspection insert, look at where the bulk of wax debris is on the inspection insert and this will tell you where the colony is clustering. If need be, you could rotate the crown board so that the feed hole is closer to the cluster.