I have this problem too, with just one user that I text a lot.
It stops working after a few days/weeks and just show the last txt in the main conversations screen, but when I click to go into the conversations with this person, it just says SMS Delivery Failed in yellow and shows me no messages.

I tried restoring the sqlite db but that did not help. The solution last time was to delete the whole conversation history for this user. What is wrong? Rebooting does not help either.