Ive just rang up to ask where my United ticket is (I bought it online last Monday at the same time as the Atalanta ones, which came last Wednesday!) and they've told me they were only sent out on Monday night, and if I dont get it by Friday, ring them up to get duplicate information! Dont know about anyone else, but Ive never had to get a duplicate ticket anywhere.
Noticed too that away tickets are only going on sale 2 weeks before games this season, rather than the usual 3, so its even more blatant when they're taking a long time to arrive.