A lot of what you can do will depend on the specifics of the settlement agreement. You should sit down with an attorney, and possibly a different one that has represented you before, to discuss the specific terms of the settlement agreement and how each party has complied with the agreement. If your ex-husband has not complied, you should be prepared to show the court exactly how you complied with your obligations under the agreement and how your ex-husband failed to comply.
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Property settlement agreements are generally enforceable through contempt proceedings which usually allow a nonbreaching party to get the breaching party to do what they were originally suppose to do by the terms of the agreement. Additionally, if the breaching party disobeyed the terms of the agreement "willfully," they will be required to pay for the nonbreaching parties' costs and attorneys' fees for enforcement.
Many property settlement agreements typically provide that the breaching party pay for costs and attorneys' fees regardless of willful disobedience as well. You will want to read over yours for this particular language.
As far as getting a property settlement agreement overturned, it is a difficult thing to do. However, it may be possible if you can allege that the agreement was so incredibly one-sided in his favor or can prove that he fraudulently induced you into signing by lying about his assets, possibly the 401k.
You should sit down with an attorney, preferably not the original one who represented you during the divorce, and discuss your options. I hope this was helpful.