A US federal judge ordered another sweep of postal centres in states that are still accepting mail ballots, NBC News reported on Thursday.
Ballots have not yet been delivered in the swing states of Pennsylvania, Nevada and North Carolina, which are still counting votes.
That includes 2,000 ballots in Pennsylvania, 800 of which are from the Democratic stronghold of Philadelphia.
But Pennsylvania Secretary of State Kathy Boockvar said a final ballot count could be available shortly.
Most mail ballots are expected to favour Joe Biden and the Democrats.
A win in Pennsylvania could give Mr Biden the victory he needs in the electoral college to give him the presidency.
NBC also reported that there are another 1,335 ballots in North Carolina that remain unaccounted for.
The US Postal Service did deliver about 150,000 ballots on Wednesday, including from the Democrat-friendly area of Atlanta, Georgia.
The service failed to meet a deadline that a judge set on election day to sweep 12 postal districts for 300,000 undelivered ballots
An analysis by The Washington Post found that almost 7 per cent of postal ballots, or tens of thousands, were not processed in time to reach election officials.
The Trump administration introduced cost-cutting measures that caused further delays in the already beleaguered system in the weeks before the election, during which mail voting became more important because of the Covid-19 pandemic.
Counting mail ballots has also contributed to the slow pace of tabulating ballots in many states after election day.
The Republican-controlled legislature in Pennsylvania, for example, would not alter a law that barred the state from counting mail-in ballots before election day.
Friday is the last day for mail-in ballots to arrive in Pennsylvania and North Carolina.
North Carolina is still tabulating provisional and late-arriving ballots and may not announce results until late next week.
Nevada’s Clark County, which includes the Democratic bastion of Las Vegas, expects to finish counting postal ballots by Saturday or Sunday.