Question : Windows Mobile ActiveSync Calendar Items do not sync

Windows mobile phone won't sync the Outlook Calendar. Mail, Contacts and Tasks
are syncing fine. There is no error. Number of appointments are shown
and seems to be downloading, but no processing. This is skipped, while
Mail, Contact and Tasks are shown. The problem is one way,
appointments made/edit on PC
are not synchronized, but appointments
made on the phone are synchronized on the PC.

Answer : Windows Mobile ActiveSync Calendar Items do not sync

Sorry - the question left me a little confused.

Is this the only mobile device / user having the problem or is this a general problem?

What version of Exchange are you using?

What Anti-Virus software is being used on the Exchange server and is it set to ignore the Exchange Structure?

It could be one of the following problems:

1. Problem with the device - in which case, delete the account on the phone and set it up again. 
2. The user account is messed up, in which case either move the account or repair the store. 
3. IIS settings are not setup properly. 
4. Anti-virus Background scanning is not enabled and this often messes up the sync.  A registry key addition can resolve this one. 

Let me know the Exchange version and I will give specific advice.

If you are using Exchange 2003 - let me introduce you to my article:

http://www.experts-exchange.com/articles/Software/Server_Software/Email_Servers/Exchange/Exchange-2003-Activesync-Connection-Problems-FAQ.html

Either way - visit https://testexchangeconnectivity.com and run the Exchange Activesync test (not the one with autodiscover), manually specify the server settings and run the test.  If the report does not pass - please report back the results.