Back in March we received a CP2000 indicating a substantial underpayment of our 2019 taxes.

Upon investigation the culprit was fairly obvious (and the form made it pretty clear if my investigation didn't), I had neglected to include a 1099-B in my initial filing.

The CP2000 showed three columns:

Shown on return: $0 As corrected by IRS: ~$46K Difference: ~46K

As well as a detail of the two expected 1099-Bs that were missing.

Ok, no problem. Hunted down the 1099-Bs, filled out the Form 8949 and Schedule D and sent it in. Certified mail, confirmation of IRS receipt. I'm thinking everything is hunky dory.

Fast forward to yesterday: Go to the mailbox and see an envelope from the IRS. Wouldn't you know it? It's a CP2000 for my 2019 returns. I compare it to the one we received in March and it might as well be a copy/paste job other than the date in the upper right corner they're 100% identical.

I (believe) that I followed the instructions to a T. I've looked this over and if I start from scratch I don't think I'll do a single thing different than I did in April. I know I'm not giving much to go on, but is there an obvious step I might have missed? I've spent a couple hours with this now and the end result is that I'm going to copy the forms I sent in four months ago and resubmit them because I can't see what else to do...but obviously I didn't do something right the first time!