iPhone to Office365 mail sync issues for certain users

We are using Office365 for enterprise email. I have several iPhone users (4/4S/5) on iOS 6.1.3 & 6.1.4 who are intermittently having email sync issues.

They have reported to me that when they try to sync email the loading wheel will spin for several minutes and they eventually get the error "message cannot be downloaded from server"

Once that happens, the iPhone *never* downloads that particular message and the user must use Outlook or OWA webmail to retrieve it.

Also sometimes happens when the user is downloading attachments, and sending messages too.

Switched them from Push to Fetch (15 minutes) - no change.

Happens on both Wi-Fi (secured) and LTE (VZ Wireless).

I have configured the same users' accounts on Android devices and I am not experiencing the issues at all on those.

I remember there were verified issues with Exchange and 6.1.x a few months ago but was *told* that those issues were resolved with 6.1.2. Now I'm not so sure...

Does anyone have any insight or guidance?

Thanks in advance!