The presence of answers, or the presence of an accepted answer doesn't matter. On the other hand, posts that have a lot of votes are generally not migrated from Stack Overflow to other sites. This is because the votes are conserved when a thread is migrated, and if a highly upvoted migrated post turns out to be bad, the recipient community wouldn't be able to cast enough counter-votes (I think they shouldn't be, precisely for this reason, but we have to cope with the site engine as it is).
There are other reasons not to migrate questions. If the question has satisfactory answers and would be a duplicate on the target site, it is often not migrated. If the question has a low quality, it is generally not migrated.
If a moderator reviewed your flag and thought that it your suggestion to migrate was not unreasonable, they would typically mark it helpful whether or not the question was actually migrated. Sometimes a moderator of the source site asks a moderator of the target site whether to migrate; if the answer is “no”, the moderator of the source site just closes the question as off-topic. It's also possible that the question was closed by a community vote before your flag reached a moderator; in that case your flag is automatically marked as helpful and filed away.
If you really think that the question should be migrated, you can flag it even after it's closed. Don't do this just because you want to get the question away from Stack Overflow. Do it if you're a member of the target site and you want to have the question on “your” site. The golden rule of migration is: “we want this question so migrate it to us”, not “we don't want this question so let's fob off the question to them”.