Why does my iPad Touch ID sometimes won't work when I place my finger correctly?

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• Use Touch ID on iPhone and iPad - Apple Support
•  If Touch ID isn't working on your iPhone or iPad - Apple Support

Some users just have difficulty using TouchID. The causes for of TouchID being unreliable for some users - and not others - are many. Whilst not an exhaustive list, possibilities include:

• Very high humidity (“sweaty” fingers)
• Very low humidity (excessively dry skin)
• Poor fingerprint ridge-detail (excessive hand washing - or exposure to abrasive material, damaging fingerprint detail)
• Physical injury (cuts or splits to the fingertip)

If you have naturally dry skin, try using a “light” (non greasy) and easily absorbed skin moisturiser.

Did you already have a look at the troubleshooting mentioned in this related article?

If Touch ID isn't working on your iPhone or iPad - Apple Support

It maybe you or the iPad. It happens.