Check for fabric caught in the closed zipper teeth and gently pull it out using a pair of tweezers. If no fabric is visible, rub a light coating of graphite pencil, vaseline, a crayon/candle ...

The first problem — a chewed-away zipper bottom — is quite common, but requires no sewing to fix. As you’ll see in the video below, all it takes is a drinking straw, some hot glue ...