IMAP folder no longer available on iPhone 6

I have an IMAP folder that is no longer available on iPhone 6.

Is there a size limit or other cause that might prevent it from working?

The folder shows up under Account, Advanced settings but is not available as a folder to sync. It also not visible to browse from email. The same folder is still working from email on my iPad Air running the same iOS 8.1.2.

This behavior started about a week ago and I haven't found a related cause so far.

Wayne