memory boot

Odd memory issue with the Prime B360M-A I've recently upgraded from 2x8gb Crucial to 4x8gb, of exactly the same type - 4x CT8G4DFRA266. With 4 Dimms installed, the PC wont boot at all, but it will with 2 installed. Installing two in A1 & B1 works, as does in A2 and B2. Also i've tried every...