When they eat, you will know it! They make a mess lol. Are they warm/cool enough, and moving around well?
You may move over to the other duck threads (if you haven't already) for other experiences with little ones. I've never had them not eat that long, but I do believe they will when they need to. If they are active and happy otherwise, I wouldn't worry too much.
Eta oops, this is in the duck section, I was thinking it was in Incubating section.
Maybe even start a new thread, or ask on The Duck Thread.