2012 15.4" hi-rez(not retina) screen, i7, 16 GB ram... I had to replace the clamshell on my MBP (2012 15.4") and now I cannot get it to charge or operate when plugged in. The charger/power cable is good- if I plug my MBA into it, it works just fine. I thought it was the battery connector being loose, but that isn't the issue. Any suggestions as to where to look for problems?