Check the needle is correctly inserted with the flat side facing the back and pushed up into the machine as far as possible.
Check if the needle is damaged or bent, if so replace it with a new one.
Check the fabric is not pulled excessively when sewing. Let the feed dogs move the fabric. Do not pull the fabric.
Check the combination of needle size, thread size and fabric is correct. Be sure to use the correct needle and thread size for the fabric you are sewing
(example: Denim-size 16 needle and 40-50 heavy duty thread).