You read about stories that they owe thousands, so when I say force it I mean one strike and you're out kind of enforcement, do some extra digging for those who claim to be self employed, take a percentage from their business
a) the CSA/CMS is highly inefficient.
b) Claiming to be self employed isn't the issue. Actually being genuinely self employed with an accountant is a massive problem. It is perfectly legal to minimize income so as to minimize child support. If it's your business, you simply don't pay it. It takes the CSA/CMS a hell of a long time to catch up with you and when it does, it is forced to use the court system for enforcement. As a result, you can run up £20k and have a judge make you pay it back at £10 a week. In a nutshell, if the HMRC accept the figures, then so do the CSA/CMS.
I also wonder what the attraction is to a man who has so little skills, dedication to work or morals that they deliberately make themselves poorer to avoid paying anything. Maybe the choice of partner has a bearing on all this
My ex husband is a professional man who knows better. Plenty of skills and dedication to work. He certainly isn't making himself poorer by not paying maintenance, is he?! And of course, had I said to him at the planning of babies stage 'darling, you will never leave me high and dry without maintenance for our children, will you?' he would have said 'Of course I will! My money is mine and I will hate you so much for having the audacity to get on with your life when I leave you that I will refuse to support our children' 
Seriously, stop with the misogyny and actually think. It is not my fault if my ex refuses to support his children. Nor is it my responsibility to do the jobs of two people to make up the difference. And yet here I am, doing just that.