RAM Boot Issues - Help Appreciated

Hey guys,

This is my first post here on Tom's Hardware and sadly it isn't at all a positive one. See today I purchased a new chassis for my PC after my old case no longer meets my demand and lacks the space required to continue my build. Whilst doing so I've stumbled upon a frustrating issue. I pulled apart my old rig in preparation for the new case and upon reconstruction I've been unable to boot at all and I am confronted with a solid/continuous DRAM LED on my motherboard (ASUS P8P67-Pro).

What puzzles me is that I've not changed my configuration or added any components to my PC since the change in chassis. I can boot with 12GB of Corsair Vengeance 1600MHz RAM, yet the moment I try to boot with my prior 16GB it get the problematic LED. I've held the "MemOK" button yet to no avail it fails to verify or rectify anything.

Any ideas as to what could be preventing me from booting with my RAM as before? As stated, nothing has changed internally on the PC, just a simple swap in case.

Any help appreciated,
Infamous Genetics.