I pay for our son but ex wife won’t pay for our daughter

My ex wife refuses to pay maintenance for our daughter who lives with me, our son lives with her but as I am employed if I don’t pay the CSA takes it straight from my employer. Not that I would never pay as I want to provide for my son but I am providing for both my children as my ex wife has her own hairdressing salon and is self employed so the CSA are powerless to take money from her, also she has declared that she earns 23k per year yet she pays rent on her shop and rents a three bedroom house has holoIdays runs a car on 23k? I do believe she is lying to the tax man!!