I think this started with iOS 9. App updates often seem to fail quickly and require multiple presses of either the
UPDATE button or the
Update All link at the top. This happens both on iPad (4) and iPhone (6S). It sort of looks like the connection to the App Store quickly fails but later works - it is unlikely to fail once the progress bar has started.
There are a few tricks that help (repeated taps of the buttons; so many taps that the app store cache clears; killing and restarting the app store; restarting the device). However, the problem keeps coming back. It seems worse on the iPad, which is sometimes left unattended for a few days or a week. Later there are many pending updates.
It also often confuses pending updates with installed updates - the button changes from
OPEN and the
Update All button is present but greyed out. For the first ~half second this is not the case, so you can still install the updates if you are quick.
This sounds very similar to iOS App Store will not update apps unless the Update button is pressed many times. However, in this case there are no failure messages and resetting network settings has not fixed the issue. Also, iOS updates never fail for me.