Is the top thread all the way into the tension disks? Try to separate them with your finger nail when putting in the top thread. Sometimes it feels and looks like the thread is all the way into the disks, but it really is just sitting where the two disks meet.

I had tension problems with my Queen Quilter18, made by Tin Lizzie. It was so frustrating. I suffered with it for almost a year. The company was no help. I bought my machine on-line and as soon as it went out the door, their customer service went down the drain. I offer this... Set aside some time and just work with it. I started with the bobbin as that is harder to adjust. Take the bobbin out of the machine, but leave it in the case. Pull on the thread. It should "bob" kind of like a yo-yo. It shouldn't pull out too easily, and it shouldn't lift the case out of your hand when you pull on the thread. When you get the bobbin set so it pulls like above, start on the top tension. Mine was way to tight. I had to loosen it almost 3 whole turns to match the bobbin tension. From that you can make little adjustments to get it exactly right. Hope this helps.