I had a problem where a contact would not sync, to the online Gmail, from Thunderbird. Turns out I had recently added that contact to a distribution list, within Thunderbird. Removing the contact from the mailing list then allowed the contact to sync to the online.
This is a known issue with the Zindus add-on. I found it easier to simply delete the distribution lists and go from there, as I very much enjoy the synchronization aspect more than the existence of distribution lists.
However, in my opinion the real solution is to create Google Groups, assign contacts to them, and then let Zindus sync those to your Thunderbird. This is effectively the same thing. The flow is simply different. In this scenario, you create the groups, in your Gmail online account, and let them sync TO Thunderbird, wheres with distribution lists, you create them within Thunderbird. Having the store (or main store, if you will) reside within the online appeals to me more anyway, as one is then able to access/maintain them from any capable device.
For more information, see the Zindus-Thunderbird FAQ here: http://www.zindus.com/faq-thunderbird/ , or specifically here: http://www.zindus.com/faq-thunderbird/#toc-one-of-my-contacts-wont-sync .