There's considerable speculation that this (delays and disappearances) of mailed ballots is 'by design'-- voter suppression techniques updated for the age of Covid-19.
It's of course entirely coincidental that Post Office budgets are being slashed just before the most critical time-sensitive mission the Post Office needs to fill: overtime being ruled out, and hours cut.
I imagine if your locale has drop-boxes, there will be issues there, too. (EDIT: i should have said 'may be issues'.) There are no requirements for them in my state, and the decision is county-by-county. My county apparently has none. If I lived in the next county over, one block away, there is one drop box, 21 miles away by car, and likely 2 hours each way by public transportation. (And I can't use that one, since it's specific to the county election office.) Easy access is apparently not a priority, or possibly there is no financial assistance to make it possible.
I might add for those who do vote by mail it would be a good idea to vote and mail their ballots post-haste to avoid the possible slowness of the USPS in processing the mail. Mailing or dropping the ballots off at the local P.O. would be an additional safety measure.
If your county / state doesn't have ballot drop boxes (we even have drive up boxes & some areas have arranged with fire houses to have secure ballot boxes - one is particularly cute in that it is housed in an old lever vote machine - I took a young nephew and that voting machine was almost as big a hit as the fire engines) then I would drop ballot at PO in the same zip code as the board of elections / ballot mailing zip if you or family member travels take you that way. Any PO I've been in has a separate slot for "local" mail. Some of the big city ones I've been in cover several zips in "local" slot.