If the first couple of stitches jam or form a knot at the bottom, I suggest you make sure that you are holding the end of your thread in the back of the machine before you start sewing and during the first few stitches. I have to do that even with my high end Bernina. After the first few stitches are secure you can them let go. Make sure you take a couple of stitches to the front, the same to the back, and then sew normally.
Maria