How old is your machine? The check spring in the upper tension mechanism can wear out and lose its springiness. (Not sure about your machine, but on mine it is the wire spring that comes right after I run the thread around the upper tension discs.) When that happens it doesn't pull the bobbin thread up the way it should.
I second the recommendation to try Glide thread. King Tut worked okay in my machine, but Glide is even better.
Also, you might want to try a needle from a different packet. Quality control on production runs of needles can vary, so you could have a bad batch.