Bandcamp records a sale under the email address used at checkout, which is not always the address on your Bandcamp account. If you paid with PayPal, or checked out as a guest, your purchases are filed under that address instead. Open your Bandcamp purchases page, or the confirmation email for one of the albums, and check which address the receipt was sent to. Then enter that address in the "Bandcamp purchases" section of your account page: you will receive a code by email, and once you type it back, every purchase made with that address is attached to your account.
That means purchases made with it are already recognised automatically, so adding it again would change nothing. If yours are still missing, they were almost certainly paid for under a different address: check the one on your Bandcamp receipt rather than the one on your Bandcamp account.
Up to an hour. Purchases made with the same address as your account here are picked up automatically by the hourly sync, with nothing to do. Purchases attached through a verified address appear as soon as you enter the code.
You can verify as many addresses as you need, one after another. Each one attaches everything bought with it, and the addresses you have already verified are listed on your account page.
Purchases are recognised album by album: one bought on Bandcamp is attached only when the very same album exists in this catalogue. A few older releases have since been discontinued and are no longer published here, so purchases of those cannot be recognised on this platform. They remain yours on Bandcamp, where your download stays available.
Albums obtained with a download code cannot be recognised here. Bandcamp only shares actual sales with us: a redeemed code is not a sale, so their API never reports it, nor who used it. The album remains yours on Bandcamp, where your download stays available.