I'll get with you on this tomorrow if nobody else does, but two thoughts come to mind right away.
One, you may have one slot that ONLY supports SATA M.2, while the other supports both SATA and PCI NVME M.2. If you are in a slot that only supports SATA, it's not going to work with an NVME drive.
Two, if you are trying to get this recognized as the boot device, it is likely there is a setting in either the boot section, or one of it's sub-sections, or in another section of the bios (Make sure you are not in some kind of EZ mode. You'll need to be in advanced mode to find any advanced configuration settings), to enable PCI device boot capability. If you are only trying to use it for storage you should not have to do that.
Keep in mind there are likely population rules regarding PCI lane allocation as well.
Probably the MOST important thing though, it to go to the manufacturers product page and make absolutely sure you have the most recent bios update installed. MANY if not MOST motherboards will not support the Samsung 970, or have issues with it, without a fairly recent bios firmware update.