The program is limited to 10,000 Address Book records, and to 100 Datebook. If you have more than the numbers mentioned here, Palm Desktop will drop them while keeping the limit. Unfortunately, there is no tweak to bypass these limits. However, there is another application called CompanionLink which is shareware, but at least doesn't have any limits and you can transfer as many entries as you have.
CompanionLink official website: http://www.companionlink.com/