Why is the d pad down arrow not working

This can be due to dust between the contact pad and conductive film, a tear in the contact pad or film, or the conductive film's contact area having gone bad at that location. You can check if the film and pad are clean and do not have any signs of tear or fading on them. Also, to check if the contact pad is still operational, you can open your controller, carefully reattach the ribbon connected to the motherboard and connect your controller to a PC and go to this site https://hardwaretester.com/gamepad

After that, you can rotate the pad in the d-pad section and check if the contact pads are generating inputs in other areas of the d-pad film. This tells that the d-pad is OK (these hardly go bad except for the ones in the triggers or if they developed a tear).

Your next step will be to buy a new conductive film for your controller. You will need to take a picture of your conductive film and match it to the ones available online. There are four types of it I think. You will need to select the correct one and replace once received.