Are there any laws governing how a company treats one owner-operator compared to another, i.e. one being fed more and better work than all the others?
Welcome to the world of contract law. There are no protections for owner-operators beyond the contract. If dispatch procedures are covered in the contract, you might have a leg to stand on, but if not, you’re on your own.
You might want to discuss the situation with your operations manager, but that might not resolve the problem. Dispatchers should be fair, but there are no laws demanding fairness.