Did you efile it from your ProSeries 2020? If so, what is the status there? Because you cannot keep sending the same thing over and over, if you see an acknowledgement and successful filing. Have you pursued it using your professional preparer contact resources with the IRS? Has the client made and used their own IRS account and that is why they know the IRS didn't get it? Are you informed because the client got a letter from the IRS?

More details always help.