My pekin duck eggs still haven't hatched and it's the 28 day?

Hi.. I have pekin duck eggs and I have had them in the incubator for 28 days and they still haven't hatched . I candled them yesterday and I saw them moving around but I'm worried because aren't they supposed to hatch today?? Thanks guys. I really hope they hatch! Is it always 28 days or longer? I heard it's 28 days exactly. Thanks