As everyone knows, we get a fair number of questions on this site that are not research level, but would be appropriate for http://math.stackexchange.com. As a frequent Math.SE user, I have an interest in seeing that they get there (particularly because sometimes I want to answer them!).
We have a "migrate to Math.SE" closure reason, under "This question belongs on another site". However, in order for a question to be migrated, we need a majority (?) of the close voters to choose this reason. Unfortunately, what usually happens is that a majority of close voters will choose one of the following two close reasons, which are usually also applicable:
"This question does not appear to be about research level mathematics within the scope defined in the help center"
"MathOverflow is for mathematicians to ask each other questions about their research. See Math.StackExchange to ask general questions in mathematics."
Both of those are true statements about the typical migration-worthy question, but choosing either of those close reasons does not cause automatic migration.
So the usual result is a question that is simply closed here, and the user either reposts on Math.SE, or gives up altogether. Either way we don't get the benefits of migration.
Maybe part of the solution is just educating 3K users about the benefits of migration and how to achieve it, but it might also be possible to reword the close reasons to help users realize that the migration option should be used when possible. The "See Math.SE to ask general questions" close reason seems particularly unhelpful since I think it actively leads people away from the migrate option.
A shining example (as of this writing) is https://mathoverflow.net/questions/209798/is-there-a-bicontinuous-bijection-from-the-reals-to-the-continuous-functions-ove. There is a +4 comment saying "This is a good question, but take it to Math.SE". It was closed with the "See Math.SE to ask general questions" reason (my vote for migration fell in the minority), so migration did not happen. I know an answer to the question and could post it right now, but I can't post it here (as the question is closed) and I can't post it on Math.SE (as the question doesn't exist there yet).
We could flag a moderator to migrate it manually, but that is a pain for the moderator. Moreover, by the time it happens, the asker may well have already reposted on Math.SE. Then there is a duplicate on Math.SE, which is a pain for their users/moderators to clean up.
How can we improve this?