I sent a journal article last year in April to a certain journal but it has taken too long to get back feedback from the editors. Should I withdraw and send the article to another journal?
Sounds like it to me. A review period of six-months, or more, is more than justifiable to question. I would advise, however, that you contact the editor first to query the delay. It may have got 'lost' in the administrative process.
Yes. Request the Editor to let you know the latest position of the MS. If the reply is convincing you can wait. There are chances that you will not receive any reply. In that case just withdraw the MS (keep evidence of the email for withdrawal of the MS) and submit it elsewhere.