Why is my mobile not charging properly?

@bidursubedi first you want to check if there is a software update for your phone available. IF there is, update it. If that does not yield any results, you may need to try a hard reset. Make sure that you have all your password, access codes and all your files backed up. A hard reset will delete all of those. Do NOT perform this if you do not have your passwords/access codes etc. Here is how to do a hard reset and if you can't do that, at least try a soft reset

Symptoms like this "charge becomes lesser and lesser" are usually caused by a bad battery. Use this guide Xiaomi Redmi Note 10S Battery Replacement to repalce it. After that re-evaluate

check charge cable and adapter then usb socket then cgarger inner of phone main board battery and battery charger chipset. one of them broken/errored.