It is showing "not charging" beside the battery
Whenever I plug it into charging it says not charging beside the battery icon but some times it does start charging.
iPad Air, iOS 12
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Your description would suggest that you are perhaps attempting to charge from a Power Adapter, or computer USB port, that cannot supply adequate power to charge your iPad. Most models of iPad require a USB power source rated at 10-12W (i.e., 2.0-2.4A @5V) to reliably charge. Older iPhone Power Adapters - and many computer USB ports, are only capable of providing a maximum of 5W (1.0A @5V).
These support pages should help with troubleshooting charging issues:
Charge and monitor the iPad battery - Apple Support
If your iPad won't charge – Apple Support
Troubleshooting generally requires substitution of the Power Adapter and cable for another known-good item.
If you continue to experience difficulties, you might be best advised to seek assistance directly from from Apple Support - and request a remote diagnostic test. You can contact the Support Team using the Get Support link at top-right of this page. Alternatively, you can initiate and manage your support cases using the excellent Apple Support App. If not already installed on your iPad, the App can be downloaded from the App Store:
https://apps.apple.com/gb/app/apple-support/id1130498044
You might also visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad, Power Adapter and cable(s):
Genius Bar Reservation and Apple Support Options - Apple