I have four users with a shared calendar, three of them use zimbra desktop and one uses the web ui. users A,B,and C use zimbra desktop and user D uses the web ui.This shared calendar has worked correctly for over a year now and has correctly synced their appointments. Now I have user A who adds several appointments to the calendar throughout the day like she usually does and these events show up on user D's (web ui) calendar but not on user B or C. This happened over the process of the day. They let me know of this issue at the end of the day and I checked user B and C's settings, they can still receive and send e-mail and every other function appears to be working correctly, but not the shared calendar. I unchecked the shared calendar on user C's computer and rechecked it. Calendar suddenly synced, this also worked for user D's calendar. I went back to my computer and looked at the shared calendar and it appeared to be updated correctly. This causes conflicts when scheduling appointments. I am completely stumped as to how this happened and would have chalked it up to a hiccup in zimbra desktop had it not occurred for two users at the same time on the same day and has never occurred before. any ideas on what caused this and how to fix it?

running zimbra desktop 7.2.1
zimbra open source 7.2.1