Should I ask a question to another Stack Exchange sites if it is not answered here at Ask Ubuntu? Sometimes, after asking a question here I found the topic is more suitable in another site. Then what should be my approach?
The best approach would be to flag it for moderator attention, asking them to migrate it to the site of your choice.
Another thing you might consider is starting a bounty on your question to get it more attention.
It is considered bad form to ask the same question on multiple Stack Exchange sites at once (and will usually get most, if not all, of the questions closed), however if you don't get an answer in a while I think it would be okay to ask it again somewhere else.