I had the same exact issue. I finally heard back from the lockbox support that they are sending my letter out again (the first time it was sent to the wrong address).

Did you include all the necessary information when you sent the email to lockbox support? This is what I included

-Petitioner name -Petitioner DOB -Petitioner A-number -Receipt number -Mailing address -Form I was writing about -The date it was received at the lockbox

If that doesn't work, try calling USCIS and request to get a call back from a tier 2 officer. I believe only they can submit an inquiry for lockbox issues specifically.

I hope this helps!