Depending on exactly what Social Security funds he's receiving, the daughter should be entitled to a benefit as well from Social Security. You should go to your Social Security office with a certified copy of whatever court order you have proving that the father is the father and ask what benefits your daughter may be able to receive as a dependent of the father. It is very common for children to get benefits as a result of their parent getting benefits. Those benefits may equal or exceed the court-ordered child support. It's very well worth it to spend some time at the Social Security office.
The reason child support enforcement doesn't collect when parents are on Social Security is, generally, that the parent has been determined to be unable to work (according to Social Security standards--maybe not your own standards). Trying to get a court to compel payment will run into the same issue--ability to work.