I've had various experiences with apps not updating and don't know cause and effect in each case, so I try various approaches:
One random thing that worked once when only my husband's apps wouldn't update, even though we share one iTunes account and the same apps updated on my iDevices: I changed the date to one day ahead or behind (in Settings) on his iPad and all his apps updated. (I googled and found that tip.)
Other times, I update individual apps, rather than using the Update All function. I think that sometimes works (at least judging by trial and error) because one app's update is somehow hanging up the others.
During other times, I restart or reset my iDevice and the updates kick in.
During other times, I sign out of the app store and sign back in, and the updates kick in.
And sometimes it's just that the app store is down, so I give it a day or so.
I've used iPad for three years, and iPhone before that, and have always managed to use of those approaches and update when the normal approach didn't work.