I use Zimbra with satisfaction for several years and have always used mobile devices like iphone, ipad etc..

However, every now and then, I have problems with synchronization of contacts.

If I insert contacts from Iphone are not synchronized on Zimbra and vice versa.
If reconfigured on the general settings of the iphone zimbra account fixes the problem to reappear after some time.

Has it ever happened?
How do I find a solution?

I'm experiencing the same problem on:

Zimbra 7, 8 Zimbra
Iphone 4 ios 6
Iphone 4 ios 7
Iphone 4s ios 6
Iphone 4s ios 7

Thanks to all