There is sometimes a TextSecure error sending message, but not always. There can be many causes for this, and TextSecure is far away from being beg-free.
This solution will only help you, if both parties are using wifi for sending messages via TextSecure.
TextSecure seems to have a problem sending messages via wifi. The cause is, that the push messages can’t wake up the wifi of the receivers phone. So, in order to fix it,, you have to wake up your wifi every once in a while. Luckly there is a small little app, that does exactly that for you. It’s called push notifications fixer, you can download it here:
Just install the app, and you’re good to go. Hopefully this solves your problem!