For those still having trouble with third party SMS apps, you can try this work around.

Why are new messages not automatically syncing? (Go SMS, Handcent and MIUI users):
Third party SMS apps such as Handcent and Go SMS can block Tablet Talk from receiving messages. This problem is easily fixed in two steps.

1) Change the setting in your SMS app's settings which prevents other apps from receiving messages:

    Handcent: Settings->Application Settings->Default Messaging application: set to "Disable".

    GO SMS Pro: Settings->Receive Settings->Disable other message notification: UNcheck.

    MIUI SMS app: disable "System priority MMS app"]

2) Open the SMS app that came with your phone. Inside its settings, disable notifications. This step is necessary to prevent duplicate notifications.