Hand made pencil / cosmetic case zipper crooked.?

iv been learning how to sew lil pencil cases. after machine sewing the sides together, i hand stitch the zipper on( no other way) the lil pouches i make are adorable but when i finish my hours and hours of hand stitching i look down and notice that the zipper is crooked with the base of the pouch. the lady who taught me how to make these kinda has the same problem. i brainstormed and thought it might be that i cut the fabric crooked but i remade it and it happened again! i wouldn't mind so much but im trying to sell these so i need it to look "perfect". anyone got any ideas??