My phone doesnt charge anymore any help?

my redmi 9 doest charge anymore after i buy it on just second hand phone store they gave 2 weeks warranty but the phone was starting not to charge anymore after 4 or 3 weeks, i tried to repair it to the repair store, they succesfully repaired it but in just some weeks more it started not to charging anymore i tried to repair it my self bcus last time i repaired it the guy who repairs it said that something is loose on my phone so i start putting stuff above of my charging board that connect flex cable and it started charging but in few weeks again it doesnt charge anymore idk whats the problem if the flex cable or charging board pls help