How to remove tire valve cap (Caravan 97)

Hi,
How do I remove tire valve cap?
I was checking tire pressure and the caps wouldn't go. Is there any special tricks for doing this? Only one of four smoothly rotated out of valve. All the others are so
stubborn, I couldn't check the tire pressure.
It's Caravan 97. My better half always goes to Wal-Mart and inflates tires for free. But I want to check them myself. I don't know what the guys at Wal-Mart did on the caps.
Thanks, Jae
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Pliers if they won't come off by hand.

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