Please refrain from making “This didn’t work for me.” comments since this is not a sure-fire solution. It was one that solved the issue for MY device (which, for the record, is an iPad mini 2 running iOS 8.3 on jailbreak) so I figure I’d share this.
About 2 days ago, all the buttons for the apps waiting to be updated in the App Store flickered from Update to Open the moment I switched to the Updates screen. Some Google searching yielded results that suggested to move to another menu, turn off wifi, return to Updates and quickly tap on the Update button next to an app immediately before it changed back to Open.
While this did work, the apps refused to update; the usual circle with a blue square in the middle that should have appeared just ended up as an empty circle that wouldn’t move. To make matters worse, after I closed the App Store and re-opened it, the apps showed they were updated. However, looking at their version number in iFile (a file manager app for jailbroken devices) showed that no update actually occurred. 
Other suggestions like turn on automatic updates, log out and log back in either App Store or iCloud, rebooting the device all DID NOT WORK.
What did work was changing the system language to another language and then back to English. It’s crazy, I know, but I verified the version number of the app I selected to test after the update and it was correct (re: most recent version).
See also: https://koukoupuffs.net/2014/10/12/problem-apps-on-ios-app-store-wont-update/ and its comments from other users for potential solutions if the above doesn’t work for you.
koukoupuffs 
I had this problem for the last few weeks since going to ios9. Did the subsequent minor updates to 9.0.2 and no better. I tried the trick of changing language to other than my original and then back and now everything is updating fine again. Thank You so much for your great site! And your readers also.