The upper thread is not inserted properly into the upper tension. Even if the jamming is under the fabric, it is the upper thread that is not set properly. ON a sewing machine, the result is opposite of the action: if you see bunching, looping or jamming under the fabric, this would indicate that the upper thread is threaded wrong.
If looping, bunching or loose thread occurs on top of the fabric, this indicate the bobbin thread tension is wrong or the upper thread is not threaded properly. See FAQ's from Europro's website