I am involved in a case with very similar facts to yours, right now. We got involved after another attorney had recommended settlement to the client and an attempt to settle had been made. In our case, this was done with a court reporter in the room at a scheduled deposition.
The court found there was sufficient agreement to enforce the settlement. We are currently appealing that decision.
What you are up against is common law and a judicial tendency, (for "public policy" reasons), to encourage settlement. You would probably need to argue mistake or mutual mistake and try to persuade the court that the omission was serious enough to undo the "meeting of the minds" of the parties. You might also argue that the other side would be unjustly enriched by having his liability released, when he is not doing the same thing.
In my view, if proper formalities were followed in procuring the settlement agreement, you have a pretty difficult case to win.
Good luck to you!
Mr. Frederick provided a very thorough and accurate response. I would just add that if you requested that your attorney refrain from submitting the signed release, he has a duty to do just that. Particularly if something was left out. If your attorney is not responding, you should write him a letter and/or e-mail outlining your concerns and requesting a meeting to discuss the missing language. However, your failure to notice the problem before giving the "OK" is hard to overcome. But since the document was not delivered, you have some hope, albeit slim.
Please be advised that the above response does not constitute legal advice. The answer is provided for informational purposes only. You must make an appointment for a consultation with an attorney for an extensive review of your situation.