How long have you had the original group? They might still be settling in and the drake hasn't started getting back to business. Though the females obviously have, so maybe he just needs more time to get back into the mood? Do you see mating behavior from him?
At any rate, it was probably a good idea to get the second drake with that many females if you are after fertile eggs. It sometimes takes awhile for ducks to accept newbies into their group, so you might want to keep the new guy separated by a fence (so they can all see each other, but no fighting is possible) for awhile so they can all get used to each other. Are there ANY of your other ducks that are ok with the new drake? If so, you can try one or two nice ones in with the new guy so he has company (and an entourage to help him fit in when you put them all back together). Good luck! 
