I am having trouble with updates also. I called apple support and they had me reinstall the latest ios update and then restore my data. It did not work. At times I cannot update on wifi and then turn around and do it on cellular data. I see no rhyme or reason of why it works sometimes and not others. It appears to not work more at night. Reseting or turning of the ipad does not help.
It did not start happening until the last update for me but apple said it is not a ios issue.
All I know is that it does not update apps about 50% of the time.