Could be that the SSD/HDD cable may be loose or damaged. Need to open the MBP again, carefully remove the connector from the SSD and reinsert it carefully, then trace the cable back to the logicboard and carefully remove the cable then carefully reinsert it. If you haven't done the latter before you should look at a video (ifixit or youtube) on how to remove the cable from the logicboard, don't just yank on it.
If that doesn't fix the problem, you probably need to buy a replacement SSD/HDD cable.