Well if your ex lied on his court papers or to the court by not listing it as one of your assets or retirement plans then I think you should be able to take him back to court on perjury charges. The only thing is you have to make sure you have enough proof and will you get more in return than what you spend in court fees?
Yes you can take him back to court if you can prove beyond perponderance of the evidence (court records will do this)two items
1. He did not legally declare to the court the asset of the retirement funds in question. A document would have needed to be filed with the court at some point during the divorce, some people do it, some do not. You can check with the court for this document filing.
2. That he, at the time of his declaration really did have this retirement fund asset.
Legally prove it and then you can take him back to court on the grounds of perjury once he is found guilty of perjury then the divorce settlement can be re-opened based on the recent perjury conviction. If at anytime he did declare this asset and you overlooked it and in your agreement in the divorce settlement that you agree to everything in your possesion and name and he in his you are out of luck