Why is only part of my contact data synched? How to force ..

I am using ActiveSync 4.5 on a Windows XP box. My device is an HTC Touch HD.

I recently had another of these dreaded "Something has changed on your device which requires a full resync ..." (or similar) message.
So I clicked again "OK" (what other choice do I have?) and my entire Outlook data got again synched to the device.

Only this time I - as I only noticed when I was looking for some phone number - only a tiny subset of my phone book got synced.
For each letter in the alphabet about 2..5 contacts were synced, all the rest (several hundreds!) are missing. What could be causing this?

Synchronization has worked before and when the device connects a connection is set up and stuff appears to be synched, but for some odd reason it seems to consider only a small subset of my contacts. In ActiveSync there are not settings under Contacts, so it can't be any filter or such. How can I force a full resync of all my contacts from the desktop to the PDA?

Michael