they don't open the mail in ballots till election day & some don't do it till after the others are counted & i suppose after the result is known the volunteers don't bother to turn up & they are stuck with a few doing it. that would be the cope answer to why it takes weeks to count a few thousand votes. the other one would be that lawyers are involved in trying to squeeze as many votes for their side as possible & are checking all the illegal votes.
United Kingdom