I have the Project Runway PC420-got it a bit more than a year ago, and have had no problems. Make sure you have the right size bobbin for that machine and maybe you need to take out the bobbin casing and clean it out. I find if I do that (I use my machine ALLOT) every couple months (or sooner if I am using quite a bunch of fusible fleece, which I have been doing for purses), I am amazed at how much lint, tiny threads, etc comes out. I keep my tension at about 3 for general sewing/piecing. If you are sure you have the correct bobbin, and the bobbin thread is wound correctly, your needle is positioned correctly, and you have the thread correctly threaded, then my bet would be a thread/dust in the the shuttle or under the needle plate. Let us know if she cooperates after all that. Also, try changing the needle if it is an old one.