Sometimes my apps don't auto update, and sometimes the reason is not clear, but all I can say is to keep checking the store manually every day or couple of days. Many people are not aware that it is also good practise to update your Google Play system whenever a new update to this is given out. This is not done on The Play Store. Check yours by going into settings and then scrolling down to About phone. Select this and scroll down to Software information. Select this and scroll down to Google Play system update. Select this, and yours will probably be out of date and it will direct you to update it. Your phone will need to reboot itself to complete the update. You may have two or three updates to work through. Keep checking until you get the message to say that your device is up to date. I am not saying this will make all your apps auto update, but keeping the Play system up to do date is only good for your device.
