that motherboard has a funky memory layout
normally they are labeled 1-4 not 4 2 3 1. and then you populate the odds or even slots. this leaves a gap in between and helps with cooling (what little they need)
Follow what the motherboad manual says for the memory slots, 1 and 3 or 2 and 4.
Secondly your sticks might be 2400MHz but unless you enable DOCP in the bios they will run at 2400MHz. you can see this by the 1200MHz in CPU-Z (DDR, Double Data Rate so its actually 2400MHz)
DOCP could be called EOCP on gigabyte. turn this on and you will get another 800MHz bump in memory speed to 3200MHz. its done from within the bios. on my asus board its on the main screen when you get into the bios.
your image of the memory tab shows they are in single channel. move one stick then check is it says dual channel.






