PC not posting with 4 RAM slots populated

[OfficiallyM-]

I'm running a MSI z370 gaming plus motherboard with 32gb of RAM (4x8gb).

Everything has been fine until today when the PC wouldn't post.

Did some troubleshooting and tried taking one stick of RAM out and it posted.

I then tried moving RAM from another slot into the now empty one and it still posted.

Tried taking that stick out and putting the original back in and it posts.

 

RGB lights up on all four sticks but the PC doesn't post - nothing on the monitor and no peripheral lights either.

 

Seems to me like none of the RAM is dead and neither are the slots, but the PC will only post with 3 slots populated?

 

Am I missing something really obvious here?

 

Any help / ideas would be very much appreciated!