How big is your sixline?
I have one that is about 3/4" long, and multiple bristleworms (the main breeder is now over 2ft long
). I have seen hundreds of the little guys flit past my sixline without him even going after them. Then again, he's about as fat as a horse (for his size).
Adding a lot of bristleworms at one time may increase your chances of getting them to establish, but you'd have to add about a hundred or so.
Other than that, add a little bit of sand every week, and try to get your sandbed upwards of 3-4".
Always adding a little bit of macro works too. I have a ton of macro living in my display, and I rarely ever get over 1ppm of readable nitrate.