Have you looked closely at the bottoms of their feet? Some folks who have used wire mesh find problems with abrasions and cuts and bumblefoot.
70%cocoa covered the wire mesh with an old towel - liquid would spill through and she could shake out and replace the towel pretty easily.
Could you remind us if these are ducklings you hatched or bought, and yes photo's would be great. Some ducklings have awkward feet that they outgrow as they mature. What is their bedding surface? Do they get any swim time in warm (same temperature as brooder, or maybe 5˚F lower) water?