I suspect that this is a semantic problem more than a collection problem, because “assignment” (as used in connection with enforcement) doesn’t normally occur in conjunction with child support, but I suppose anything is possible, and EWOs (earnings withholding order) occasionally go unpaid, rightly or wrongly.
Assume a properly-served EWO. That creates an execution lien for the money that the employer should have paid to you via remittance(s) to the Sheriff. You can enforce your rights informally (e.g. by discussion with the employer to confirm that the debtor is still employed, that the employer has been served, that there are no prior EWOs etc.) or formally (e.g. by letting the arrearage mount and filing a Creditor’s Suit against the employer (CCP §708.210))
Recognize that there are pros and cons to waiting until a creditor’s suit becomes worthwhile. Recognize that, if there is an existing levy for up to 25% of the debtor’s earnings already in place, yours cannot kick in until the prior levy is satisfied. Recognize that child support orders can theoretically last a very long time (longer than a judgment, without renewal.) Before chasing any or all of these considerations, when they may be irrelevant to you, more information is needed. Have you spoken to the employer? What was said? It’s very unusual for the Sheriff to venture an opinion that your levy should have been honored. Again, why and what was said?
Most of all, recognize that all of the other factors are subordinated to the debtor’s claims of exemption. If between the child support obligation and enforcement of your rights, the debtor is not able to recognize the “basic necessities of life", you’re going to wait until he can or, at best, be satisfied with a very small and slow recovery