Why is the slider getting jammed on invisible zip?
Check that there is sufficient clearance for the slider to pass along the length of the zip. Check that no side seams interrupt the travel of the slider along the chain. Check that the edge fo the coil is not stitched to the base tape of the zip.
Why is the open end part of the zip found to be broken when returned by the end customer?
In long centre front applications the end of the zip can be placed under high strain. Check that the end consumer does not create too much pressure to the bottom end of the zip when wearing the garment. If the application is a long coat a two way zip maybe required.
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.