Hi There
One of the problems that can affect active sync is if you don't have collaboration data objects installed for outlook. There is a knowledge base article here:
http://support.microsoft.com/default...b;en-us;299625
Although it refers to active sync 3.1 I was referred to it when I had sync problems and installing these seemed to resolve the problem. I have also found that reliability has improved since I installed active sync 3.7. I don't seem to get the duplicate calendar entries that I got before.
You may need to post more information about your version of Outlook; OS and version of active sync if you need more help