soc_puppet is correct -- imports are by journal, not by "account that posted this". So, people who import their personal journals don't have community posts they've made come over, and community maintainers who import their communities will only have posts from that community, etc.

If you're the admin of a community, you can import the community into a DW community. You need to be the maintainer or owner of the community, though. If you aren't, you'll have to manually copy over anything you want to keep, or convince the maintainer to import it.