When replacing the Pencil tip, ensure that the replaceable plastic pencil tip is snugly screwed into position (i.e., finger tight) into the body of the Apple Pencil; when fitted correctly you should see a tiny gap, of about the thickness of a sheet of paper, between the tip and pencil body. If slightly loosened, or overtightened, the Pencil may not work properly, if at all.
Be aware that if dropped onto the Pencil tip, the transducer located beneath the user-replaceable tip can be damaged. The transducer is the element that interacts with the iPad. If the transducer has been damaged or broken, the Pencil will pair and charge - but will not be recognised by the iPad screen.
This support page may be helpful with pairing and use of the Pencil with your iPad:
Connect Apple Pencil with your iPad - Apple Support
If you continue to experience difficulties, here is one of the better troubleshooting guides:
https://appletoolbox.com/apple-pencil-not-working-heres-our-troubleshooting-guide/
If you need assistance, the technicians at the Genius Bar of your local Apple Store or Apple Authorised Service Provider (AASP) will be able to test and assess your iPad and Pencil:
Genius Bar Reservation and Apple Support Options - Apple