The needle may not be "seated" properly( pushed up all the way to a built in needle stop). Remove , get a fresh needle. make sure the needle is pushed up to the stop. Rethread entire machine ( including bobbin) , following the owners manual. Just an FYI my machine tension is 4, so you may have to play with the tension to find out what is best.
Also if you have broken a needle , a small bit of it may be stuck in bobbin area somewhere.