When I first started sewing, I didn't know much about zippers. I bought the first ones at JoAnn Fabrics and discovered that there are quite a few varieties, and almost everything I bought at the first and second round turned out to be a little bit wrong. You have separating, non-separating, one-way, two-way, and then different sizes. There is metal tooth, plastic tooth, and coil.
They are mostly good enough, but you just have to get the right type with the right length with the right color and right size.