I lost my job and started a new one last year in 2010. My ex wife told our teenage daughter in 2009 that she was going to start full time at her job and that she was going to be making more money. It took me a while to financially get on my feet and I would like to change the order now since she get's mad every time I am late a day or two on child support.
If wife wasn't working full time, you should have asked for income to be imputed. Even if she's not working full time now, you can ask for income to be imputed. Basically, take her hourly wage and multiply by 2080.
You can ask for a modification. There are two routes:
1. Through court. You can probably do this yourself, but if ex hires an attorney and you don't have one, it can get a bit uncomfortable. Getting a court hearing may take a long time or not - depending on the backlog. Court will cost you a little money (or a lot of money if you use an attorney).
2. Contact your state child support enforcement department and open a case with them (if you don't already have one). Ask for a review. This may take even longer than the court, but won't cost you much. I'm not sure if CSED can use an imputed income - you might need court for that if Mom isn't working full time.