Best setup for binding

[nikpalmer]

I’m binding an edge on a practice bag. The leather is 1.1mm and the edge of the bag has three layers so around 3mm. I then cut a 20mm wide binding strip. I glue the bag edge up and then glue the binding strip in place around the bag edge. Then I sew (techsew 4800) the binding all around the bag edge circumference. All works but I can’t keep the stitching on the binding to be consistent distance from the edge of the binding. I’ve included a Coach bag image with a very similar shape and edge binding. Their stitch offset is almost perfect throughout the circumference of the bag, even over the places where the binding for the top edge of the bag joins into the outer edge binding. 

So I am wondering whether I am approaching it incorrectly from a machine point of view. I’ve included the foot setup image (I’m using an adjustable roller guide but it’s folded away in the image), and a couple of images of where the binding worked well and was even.  I can either see from the inside edge of the binding/bag or the outside. What tends to happen is the back of the stitch is moving laterally even if the front of the stitch is parallel to the edge of the binding. To me this feels like the whole edge and binding is “rolling” possibly where there are inconsistencies in the material thickness? 

I searched around and can’t find much on this style of binding on a machine.  Should I use a different foot setup? More foot pressure? 
 

This may be more appropriate for “how do I do that” but I’m thinking it could be machine specific so posted here.  
 

thanks for any advice!