I finally found a way to resume an update to a "stuck app" that failed to update via the normal method. My symptom was that I had 17 apps which had an UPDATE icon when viewed from the UPDATES tab of the App Store app. They would not respond to an UPDATE ALL button click and they also wouldn't respond if I clicked on the UPDATE icon to their right. I'm using an iPad Mini3 with IOS 9.1.
Here's what I did.
First I logged out of my iCloud account (Settings -> iTunes & App Store), then logged back in. I assume that caused all pending downloads to go to some other state.
Then, for each icon in my list which was failing to update before, I did the following sequence (it worked for all, one at a time):
1) I tapped the UPDATE button (this showed a completely-filled circle but didn't continue the update)
2) I tapped the completed-circle icon (this presumably stopped the update, returning the icon to UPDATE)
3) I tapped the UPDATE button again (which started the download from the beginning, and ran to completion with a successful update)