Are you knitting your tail in with the first row? Maybe you should try without.
That looks like your slip knot hanging there. Do you knit it?
Another thought--instead of using a slip knot, just drape the yarn over the needle before you make your first cast on stitch. You'll get two in one for this cast on and if the slip knot is the problem, you'll eliminate it. If it's not, then you can maybe focus on something else.