If the charging cable won’t properly seat or easily fit into the charging port of the iPad, this might indicate that either dirt or lint is obstructing the plug, or the plug or charging socket have been damaged. A damaged charging cable must be replaced - whereas a damaged charging port must be repaired.
If the problem is due to a visible obstruction, you may be able to remove it yourself*. With good light, using a sharpened wooden matchstick or plastic toothpick, very gently try to dislodge a visible the obstruction from the socket. Be careful to avoid damaging the delicate electrical contacts within the socket. Note that a USB-C socket has an obvious "tongue" within the socket.
(*) Removal of an obstruction is often best entrusted to the technicians at the Genius Bar of your local Apple Store or Apple Authorised Service Provider (AASP) - where the appropriate tools and expertise are available:
Genius Bar Reservation and Apple Support Options - Apple
If the charging problem is not mechanical, these support pages should help with troubleshooting battery and 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 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