Computer stays on when trying to put it to sleep

Whenever I'm trying to put my PC to sleep, the screen goes blank but a power indicator stays on and all fans keep on working.
I have a NVMe SSD installed on a PCIe card, and OS is installed on this disk. This problem is only happening when I put it to a PCIe 3.0 slot; when I move it to a PCIe 2.0 slot, the problem goes away but the disk works at half max speed.
Any ideas how to fix this problem?