I could write a few pages on possibilities and scenarios for low/no cells built.
But no sense in that until a few questions are answered.
I key in on your partial comment about "But they also have a frame of honey". I see this as a non-issue, except perhaps indicating the cell builder is way less than it should be. Cell builders many times are full size hives compressed down into a smaller unit to overload the builder with bees. But it should have ample honey, pollen, etc., on a scale of being more than one. You also mention putting on a feeder. (which I usually do not in a flow) Feeding is best served by doing this for the week prior, or at least a few days in advance of the bars being introduced. Putting on the feed at the same time as you put in the bars, is not time enough for the bees to prep properly if needed. Bees do not react that soon.
Can you describe your cell builder? How many frames, setup, stores, etc. How long queenless?
Low counts, can be for many things. Seasonal (we are after the summer solstice which is a huge thing), dearth, etc.
No cell counts can be due to other larvae being chosen from their own comb ignoring your offerings, a second queen in the hive, bad eggs being selected with viral issues, really poor grafting, over heating or cooling the larvae while grafting (are you wet or dry grafting?), and many other things.
Double grafting or having the bees polish the cups prior to grafting would change your take rate, but I do think that would be reason for none at all. There is something else going on, so don't focus on the small stuff yet.
For the record, my grafting rates are down this year also. Probably for a host of reasons.