Why does my site work without www but not with www?

Each version needs its own record, so one of them is usually missing or still pointing at your old host.

On the Domain page both versions are listed with their own label. If one says "Waiting for records", add the record shown for it (it's in the same list as the other), and if a record with the same name already exists, such as an old www CNAME, edit it rather than adding a second one. Wix and some other hosts leave old @ and www records that need removing first; the steps for your DNS host say so.

Once both are connected, the one that isn't your main address sends visitors to the one that is.