Loosen the top tension all the way and use a sturdy piece of thread to "floss" between the tension discs. Sometimes there's a piece of thread stuck in there that'll mess with ya big time.
Another thing: When changing the top thread, NEVER pull your thread out backward. Cut the thread at the top, release the tension (lift the presser foot), and pull the thread out at the needle. I learned the hard way...it got tangled inside somewhere and cost me a repair visit! I'll always be careful for the rest of my days