That certainly could be the problem. Memory types (TLC) ( MLC) and controller types have a great part to play in compatibility with specific motherboards. Certain types will work and others not.
What is the brand and model number of the first two SSD's?
The following websites show SSD disks they they guarantee to be compatible with the HP Z420 Workstation
https://www.mrmemory.co.uk/ssd-upgrades/hp/workstation/z420
http://www.kingston.com/us/memory/search?DeviceType=1&Mfr=HEW&Line=Workstation&Model=76981
http://www.userbenchmark.com/System/HP-Z420-Workstation/3128