There's bound to be some ballots that were undelivered. Most of the people whose ballots weren't delivered won't check and see their votes weren't counted. Perhaps some will, but these folk may not be interested in or capable of publicizing this fact.
.....
Sure, some ballots out of many millions were probably lost, maybe even after they were scanned, maybe even after they got where they were supposed to go. But the fact that 300,000 ballots were scanned in but not out doesn't mean that 300,000 ballots were lost or worse by the USPS.