Best way to get to the Taxpayer Advocate is through a Congressional.  You have two Senators and a House member.  Pick one (look for Finance Committee or Ways & Means, although if your client is a donor that might help).  

Were you tracking the 2019 refund until it dropped off "Where's My Refund"?  And now, some 2019 returns are showing up there again, if no 2020 refund has been processed. My guess would be that one or more of the ES payments for 2019 were not credited correctly.