Hi Vinh --
You posted the question awhile back, so you likely have moved on from this mac... but just in case it's sitting on shelf (like I do) when I hit a brick wall with an item I either haven't figured out how to repair or can't afford to fix just yet (due to mostly having too many projects in the works. lol), here's some input.
1. take a quality heat gun with a focusing cap (tunnels the air into a smaller opening) and go over the RAM with the gun on a high setting. Be consistent with your sweep, holding it roughly half an inch from the board or so. Be careful not to actually touch the gun to the board. The silicon/resin/etc of the board won't hold up to direct contact from the heated metal of the gun. Also, you might want to put the board on a surface that can handle the heat, or like myself, on a suspended metal tray that can't heat and burn anything underneath. Do this a couple of times until you're satisfied you have sufficiently heated up the RAM and the solder connections underneath. Allow it to cool (20 min should be more than enough). Reinstall and see if you repaired the issue. There is no guarantee, as the RAM iteself may be damaged or defective, but there is a chance you will repair the issue (I've done this multiple times and had roughly a 50/50 success rate.
2. If you can't fix it... and still want the computer, I know quite a number of very reputable certified vendors that can rework the RAM for you at varying prices. ($120 - 300). Feel free to message me as I don't want to give free "air time" advertising for repair shops. This is not the place for that. LOL. I started doing the rework of my own boards a few years back (with a learning curve) and it's been fun. But if you don't want to risk that and don't have the specialized equipment, rework from a professional is highly advisable.