What Mysticial said is correct. The question is migrated if:
- Four users voted to migrate it to the same site
- The question is using at least one tag used in the destination site
- The OP must not have a question-ban in the destination site
Failing one of the conditions, the question can be flagged to be migrated. Keep in mind that not all the questions are migrated. The first rule for migration is, "don't migrate crap"; it means that, e.g., a not constructive question will not migrated. If then the question is on-topic on the site where it is asked, it is probably the moderators will not migrate the question, if it is not the OP asking for the migration.
Also, not even moderators can migrate a question that is 30 days old. This is a recent change that has been introduced to avoid migration "hot potato" style.
This is not a bug; it's status-bydesign.