I had this problem. Tried resetting phone and jail break again, put apps on and still had the problem, and after numerous restarts still had the problem. After comparing 3 jail break phones side by side and 2 had the problem, one didn't we started looking for what the 2 had for cydia apps the one didn't. We found what was causing our issue. The 2 both had a app called copic.
Once we removed copic, turned the phones off and on iMessage worked immediately again. So if you have copic on your phone, follow steps above and it probably will start working again.
Copic is a app that puts your contact pictures in the message section in your text message. First thing I would check if having this issue is apps that change your text settings on the phone.