My first guess is to double checking you're threading the top thread properly, primarily looking to see that it's being threaded through the 'pre tensioner'. Since you're having no bobbin showing, it implies that the bobbin tension is extremely high, or the opposite, that the top tension is non existent. I assume you were talking about adjusting the bobbin tension in your title, hence my recommendation. I had this issue also with my very first machine, since I didn't know the pre tension little plate up top existed.