Shared calendars do not automatically refresh

[digm]

We have recently noticed that shared calendars on iOS do not automatically update. Here's how we set it up.
INITIAL SETUP

User 1

1. Creates a calendar called "Shopping List"

2. Shares calendar with User 2
User 2

1. Accepts shared calendar invitation

2. Configures new CalDav account iPhone and disables SSL

3. Shared calendar shows up in iOS Calendar

4. Initial list of events in User 1's shared calendar shows up
PROBLEM

However, if User 1 adds a new event, that new event never shows up in User 2's calendar. Same goes if User 1 deletes an event, it never refreshes in User 2's calendar. The only way we've been able to force the calendar to refresh is to (1) reboot the phone or (2) disable/re-enable the calendar in the Mail, Contacts, and Calendars settings.
Has anyone experienced this issue and/or have a solution/workaround? We are running Zimbra 8.0.6_GA.5922.NETWORK and iOS 7.04.
Thanks in advance!

[fooz]

My experience with a single shared calendar is that sometimes the events show up, but sometimes they do not. If they do not, the only fix is to disable/re-enable the calendar in the Mail, Contacts, and Calendars settings on each iPhone.
There doesn't seem to be a repeatable set of steps that gets the calendars on the phones into this state. I've tried:

- Creating calendar events on the iPhones, after a calendar refresh the events typically show up on both phones.

- Creating calendar events on Webmail, after a calendar refresh the events typically show up on both phones.

- Modifying calendar events on the iPhones and Webmail.
Within the Webmail calendar account settings, I have the following setting enabled: "Enable delegation for Apple iCal CalDAV client"
Release 8.0.7_GA_6020.RHEL6_64_20140321160526 RHEL6_64
Both iPhones are both running iOS 7.1.1.