Try slots 1/3 instead of 2/4 (if cpu cooler size allows) just to see if it makes a difference. Also, update your motherboard bios. There have been issues with ryzen compatibility with various memory frequencies and these have largely been addressed with series of updates by the mobo manufacturer's so make sure to keep up with these.
For example notice that two latest bios for your motherboard, include mainly memory compatibility fixes:
https://www.msi.com/Motherboard/support/X370-GAMING-PRO.html
Follow manufacturer's instructions carefully on flashing/updating bios. Done incorrectly it can brick your motherboard so be sure to be careful.