Sah48.....that's exactly what I'm experiencing with my set up. Do you know what they did for an "adjustment"? I've read that others had similar issues and after an adjustment , much better. I wasn't so lucky yet.
I still have the original pictures along with the last ones that I sent with my seat. I'll see if I can see something different with them.I want to help Russel help me get fitted properly, its not going to easy since I'm so far away.
I have the vinyl seat covering and it is quite easy to slide on but if I was planted in the pocket like I should be, it would take a lot to slide forward I'm thinking.
I also have Heli-Bars.
I am not sure exactly what they did but from the feel I would say they removed some of the rear material so the pocket was better for me to fit into. I am 5'9" and have always run with the seat in the middle position but have found for this one it works best in the low front and lowest rear settings. I also slide forward slightly at stops.
good luck!