Legal advice re: non-payment of child maintenance

NRP was unemployed paying £5 per week for 3 children. Had a letter out of the blue stating amount was going to be going up considerably, presumably because he was working. Was then told he had registered as self employed and earning £100 ish a week. No payment has been made since September 2017 and CMS have warned me that I could be waiting another 8 - 12 weeks and even then, no guarantee can be made that I’ll receive anything. The debt accrued is in excess of £3000 and I’m wondering if there is anything I can do legally?? All my local solicitors have said they don’t deal with child maintenance cases. So frustrating.