Plaxo has limits on the record types you keep online.  If you reach one of Plaxo's synchronization limits, the sync process will fail.  These causes include, but are not limited to:

Comcast Webmail
Combined address book folders have over 10,000 contacts.

Google Mail
Calendar folder has over 10,000 events.

Hotmail
Address book folder has over 999 contacts.

Plaxo for Macintosh (OSX)
Combined address book folders have over 10,000 contacts.
Combined calendar folders have over 10,000 events.
Combined tasks folders have over 10,000 tasks.
Combined notes folders have over 10,000 notes.

Plaxo for Windows (Outlook, Outlook Express, Vista Mail)
Combined address book folders have over 10,000 contacts.
Combined calendar folders have over 10,000 events.
Combined tasks folders have over 10,000 tasks.
Combined notes folders have over 10,000 notes.

Yahoo
Address book folder has over 5,000 contacts.
Calendar folder has over 10,000 events.

Learn more about best practices for keeping your address book information within the synchronization limits.