Hi, I'm Pete. Welcome to JustAnswer! I'm reviewing your question now, and will post back with your reply ASAP.
There are two possible causes of this:
1. There is debris or dirt accumulated under the zoom dial which is causing it to stick when you try to swivel it.
Try cleaning around and under the dial.
2. The dial itself has a fault or a broken part underneath.
If this is the case, the zoom dial switch will need to be replaced.
Cleaning underneath the dial resolves the problem of a sticky dial in most cases.
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Sometimes applying a tiny drop of lubricating fluid under the ring can help it move more smoothly.
Also try moving it in both directions several times to help free it up.
If that doesn't help, then the camera will need to be disassembled to clean under the dial thoroughly.