I really sympathise with you, sorry this is a long post but I hope this info helps.
My ex is a company director of a firm he part owns, he owes me five months arrears, as you say the CMS have done nothing to retrieve this money even though they could issue an enforcement order. Now he has sent wage slips to them showing that his income has diminished considerably. Basically I will get nothing moving forward not even my arrears as he isn't earning enough this year to justify the calculation for this year which was based on last years HMRC figures which included his dividends. I know he is committing fraud but the CMS accept wage slips at face value and do not have the right to cross check with HMRC until the end of the tax year. My ex is PAYE, so how easy would it be if the CMS could just check what tax he is paying!
Anyway what you need to do is to issue a mandatory reconsideration with regards to the amount you are currently receiving which does not include his bonus. The CMS will reconsider their decision but will give you the same answer as before, there is no evidence to show he receives a bonus.
Next you need to request an appeal form from the CMS, this will entitle you to go to a tribunal. This is a legal process and the judge has the power to request full financial disclosure, they can request bank statements, details of investments, etc. You do not necessarily have to attend this tribunal, you need to produce a statement about why you believe he gets a bonus etc. You do not need to get a solicitor either, take it from someone who has represented themselves a solicitor is an unnecessary expense especially with this court. The judge can also request tax details from HMRC, I'm also given to believe that these tribunals have accountants and tax experts who are well versed in all the dodges that Fathers employ to reduce payments.
For anyone out there who like me has an ex who is deliberately hiding money through leaving dividends in a company account or possibly getting another person to draw down his dividends, this also will be investigated by the tribunal. Unscrupulous individuals such as my ex use a process called a dividend waiver, this means they don't get paid their dividends but leave them and draw them down at a later stage, often years later, he is doing this as our daughter finishes full time education next year, so thinks it won't be used as part of his CMS calculation. It is illegal to use a dividend waiver unless commercially a business can justify it, this would essentially mean the business was in financial difficulties, his is not. If you also believe that this is what your ex is doing you need to explain that in your statement to the tribunal.
I would also say if the Father is self-employed and you know that they are likely to cause issues around maintenance do not opt for the family-based option, go for the statutory option, this will save you a lot of time and anguish.
Going through the tribunal process is time consuming and frustrating but I would urge you to do this. Also do write to your MP, and if you are unhappy about the CMS complain to them in writing, Citizen's Advise have the process on their website. Finally go onto Gingerbread's website, they are currently campaigning for changes to the law on child Maintenance.
Good luck, I'm just starting the tribunal process but am determined that he will not get away with what he is doing, my daughter has done nothing to deserve the poverty he has inflicted on her.