CSA arrears that husband didnt know he had

please can someone help we have just received a letter from CSA saying my husband is due arrears from 1997 to 2001 for his 2 oldest children, he still pays every month for his other 3 kids never misses a payment. We really dont know where to start to get advises. I also received a letter saying thats the CSA have writing off my ex-husband debt because change in his circumstances his arrears are about 12 years old. how come they are letting my ex off with want he is due but chasing my husband for debt he didnt know about least he has paid for his kids ....