I'm modelling a t-shirt, but I get these gaps between the meshes that are being sewed with Sewing Springs. It is as the springs aren't tight enough, but if I adjust the Sewing Force nothing really happens in term of the gaps.
I had this issue and found that the problem was not having enough sew edges. ie add loopcuts to the parts that are being sewn together and connect them so that you have more stitches. once you have enough the seam will pull fully together. (this fix assumes that there is enough "fabric" as in the gap is not caused because the fabric is not stretching around the model)