Question Primary M.2 slot not showing Evo 970, second slot only runs it at PCIe Gen 2 x 4

Dec 29, 2020
So I've just got a Samsung Evo 970 1TB drive as my existing boot ssd is full. As far as I can tell from my mobo manual, (MSI b450 gaming pro carbon max wifi atx am4) it should run in the primary M.2 slot at PCIe Gen 3 x 4, giving me the full read and write speeds for the drive. When I put it in this slot, it doesn't show in the BIOS, drive management or anywhere else. It works fine in the secondary M.2 slot however only at 2 x 4 which gives me about half the speeds I was expecting, as per Samsung's advertised speeds.
Any ideas on why it isn't recognised in the first M.2 slot, or if its possible to run at gen 3 x 4 in the second slot?

(from manual
AMD® B450 Chipset
y 4x SATA 6Gb/s ports
y 1x M.2 slot (M2_2, Key M)*
Supports PCIe 2.0 x4 2242/ 2260 /2280 storage devices
y 1x M.2 slot (M2_1, Key M)**
Supports PCIe 3.0 x4 (1st, 2nd and 3rd Gen AMD®
Ryzen™/ Ryzen™ with Radeon™ Vega Graphics and 2nd
Gen AMD® Ryzen™ with Radeon™ Graphics) or PCIe 3.0
x2 (Athlon™ with Radeon™ Vega Graphics) and SATA
Supports 2242/ 2260 /2280/ 22110 storage devices
y 2x SATA 6Gb/s ports**
* M2_2 slot will be unavailable when installing device in any one of PCI_E2,
PCI_E3, PCI_E4 and PCI_E5 slots.
** SATA5 and S)
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