Why is my mail not displaying after ios 9 upgrade?

This problem occurs several times each day, generally after I've trashed all email (leaving the inbox empty). When new mail arrives, the number of unread emails displays correctly on the mail icon, but when I open mail, the inbox is empty (I can check email on my laptop and see that new messages have actually arrived). The only way I've found to view the mail on my iphone is to reboot the phone. Once rebooted, the mail displays correctly. I've read other posts about mail issues with ios 9, so I assume this is related. Any thoughts on how to correct? Is Apple working on a fix?

In perhaps a related issue, I've noticed that my battery usage has increased since ios 9. When I check battery usage, Mail is generally one of the top three battery usage apps, most of that usage for "background activity." Doesn't make sense to me that Mail should be generating such heavy usage. Thoughts?