Recheck for a queen! How long queenless? Hopelessly queenless, meaning no young brood in hive that they could make cells from. Some folks get better results by letting the bees polish the cups overnight. Make sure to never let the larva dry out; cover the cell bars with a damp towel or if you're taking a long time to graft then use a little drop of plain yogurt to prime the cells, about the same amount as the Royal Jelly puddle the larva are laying in. The larva size should be almost as small as an egg. Make certain to have a full frame of pollen next to the cell bar frame and mucho BEES; thousands of nurse bees; bees boiling out of the hive when you lift the top; a gallon of bees hanging on the front entrance. Oh yeah, almost forgot, did I tell you to have BEES? Keep adding capped brood frames to keep the nurse bee population high. Don't know if the weather has anything to do with it but we've been having low cell starts too; temps in the 90-100 range.