--Some times you may have to use a neelde with a larger eye so that the thread will flow though the needle easier,
-Sometimes the clearance on the bobbin case stop has to be adjusted to allow for thicker thread,
-Be abundantly sure the top thread is through the discs on the top tension and not behind the tension devise, then increase the tension to a tighter position. if there are numbers on the tension, og to 5-8 so tyhat the thread will pull up into the fabric and not form loops under the fabric
-What type of thread are you using, possibly try a thinner thread
-check the bobbin as it spins in the bobbin case and after you have put the thread under the tension spring on the bobbin case, be sure the thread will pull out of the bobbin case relatively easily if not loosen the screw on the bobbin case ( righty-tighty/ lefty-loosey)
if all this fails you had better take it to an industrial sewing machine technician