Calendar updates randomly

[4610silbro]

Good evening
I'm having a Problem with my Zimbra Server 7.2.0 running on Ubuntu 10.04 LTS 64 bit.
My mobile phone downloads the calendar, email, contacts. When I make a calendar entry with my mobile it takes forever to sync. Very random it seems (up and down). It works fine with Outlook. I tried my LG P990 android phone and a HTC. Ports are open (443, 25). What could the issue be? Thanks for any advice :)
cheers

Silvio