Managing important mail

When I was home for the holidays, I signed up for the USPS informed delivery service for my mom's address so I can see when she should be receiving important things and also when to let her know a scammy thing is arriving (those car warranty cards). I noticed it isn't fully accurate on which days things arrive, but it was pretty close. So I've been emailing her on the days she gets something important and telling her where to file it. She's supposed to be collecting all her tax forms in one place so my uncle can find them to handle her return.

Today she called me to say she has never received about 5 of the tax forms I told her should be arriving. This is mail from days ago. She didn't say anything at the time. What are the chances someone is stealing her mail vs. she forgot she received them and put them in a weird place? I feel like the second is more likely. So I reminded her of all the different places she likes to throw her mail and asked her to please check them all. I think that annoyed her, but I don't know how else to help her manage her mail from so far away.

My mom has mild dementia. She has refused in-home help. Even though I listed this as a benefit to the help.