Why is the zip curved/humpy after insertion?
Check the sewing tension is equal on both sides of the zip. If this is only noticed after pressing heck for any high levels of shrinkage in the fabric material and any interlining.
Why is the zip curved after stitching continuous zip chain to the garment?
Spiral/coil zips have an S twist and Z twist in the teeth creating a “smile” or “frown” appearance when the teeth are joined. The left and right teeth should be both in the same direction/condition. If this is not the problem check the sewing tension is equal on both sides of the zip.
How do I stop the teeth creating an impression on the front of the garment’s fly panel?
Change the zip for reversed coil type zip and slider.