Recently I came across a problem whereby a blackberry users handset would synchronise with all elements of exchange except for their calendar. Below are the steps I used to troubleshoot the issue and an additional step as a last resortt, which thankfully I did not have to use!
Step 1 > Go to Calendar on the handset and then options. In the options menu check that Wireless Synchronization is turned on.
Step 2 > If the above fails, In the calendar switch to Events view. Customise the view by pressing Filters button, go to Advanced, and delete the filter. Go to the Events list, which at this point should show everything in the calendar. Look out for a yellow icon in the left hand column. This indicates that there is a conflict. Double click on the yellow icon to resolve it. Perform the same steps with the Address book.
After performing the above do a full reboot (remove the battery and replace) and leave 30 minutes or so for syncing.
Step 3 > If the above 2 options fail, go to Settings/Tools > Options > Advanced Options > Service Books. Find the Desktop[CICAL] service book. Resend this service book (or get your BES admin to do it!). If you have desktop manager available, simply plug your blackberry in and it should pull down automatically.
Drastic Option 1 > Wipe the device by going to Settings/Tools > Options > Security Options > General Options. Click and select “Wipe Handheld”. Once this has finished do an Enterprise Activation.
Drastic Option 2 > Perform Drastic Option 1 but before activating/Turning wireless on, delete and re configure/setup the account on BES.
Hope this helps!