Is the wheel difficult to turn?
If it is, I might have your answer for you
Typically, if it is very difficult to turn and you get small or no sparks from it, the flint is likely just wedged up against it too tightly.
You can rectify this by loosening the screw on the bottom that holds the flint in, never have I had a zippo that I used that it was anywhere near close to all the way tight.
If it still doesn't work, leave the screw a bit out of the hole to see if it will spark better than... if it does, you may need to adjust the spring.
Hope that helps - I had the same problem before
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