Your dresses are very pretty, and on you they are Terrific!!
I sought advice from a seamstress, and here is how I tackle that issue now:
"Machine baste the seam closure using the longest stitch possible, next lay the zipper on the backside along that closure with the zipper teeth down the seam line, and then HAND BASTE the zipper on the very outside edge of the zipper. The seamstress told me that it is very important to stitch DOWN ONE SIDE, then to stitch UP the opposite side." (Apparently this was the problem). After that advice my zippers always look great.
I hope this helps you.