My phone is not charging after 75% charge
What phone do you have?
Are you using the original charger and cable?
If it has been like that for a while, then I would reach out to OnePlus Customer Support. They can provide the necessary guidance to resolve this issue for you.
I bought a new charger from the market, but the one was just before me
Using stock Oneplus cables and chargers?
Unboxing Challenge: Your first look at the OnePlus 8T
by Crystal Z., 2020-10-22