Google Voice occasionally seems to have an issue where its database gets a bit out of kilter. My suggestion, which may or may not help (but it did for me once in a similar situation) is to go into your Google Voice account and simply toggle a few settings - change them to the opposite of what they are now, save the configuration, then toggle them back to the original values and save again. In your Google Voice destination, uncheck and then re-check Google Chat. With any luck, you'll fix the problem. No, you shouldn't need to do this and in one respect it makes no sense that this would work, but as I say, it did for me once when I had a similar issue.