I want to get what is rightfully owed

For many years I tried to get CSA for my oldest son but was unsuccessful lack of information I felt the CSA did not do much about my case so because of this I struggled as a single parent and my son lost out financially.

He is older now but I still want to get something for him as I claim arrears for my youngest son and the amount I get for him is shocking as his father also went years with not paying only I was a bit wiser and fought hard to get some money.

My sons father has never given me anything so please can you help or give me some advice on how I can retrieve some of thousands of pounds that is owed to me rightfully.