For $29/mo, we provide access to advanced level technicians who will answer your questions on any Macbook board related matter to the best of their knowledge promptly & walk you through how to solve your problem so you can deliver a working board to your customer.
Drop or someone forcing RAM in/out is the most common. Shitty soldering on the BGA RAM slot can of course also happen. If you have a RAM slot tester you can check which data line is open and see if you are lucky that it may just be a stuck pin from minor damage to the slot. That can usually be freed up.