Forget trying to convert PCI to PCIe.
Given the slots are recongized by the OS and assuming no HW faults in the bridge or riser, power seems the most likely problem.
You must use powered risers. The host x1 slot may be limited to 25W where x16 can supply 75W. Even if the slot does support
75W that has to be shared with all three slots on the bridge.
You could try connecting the card directly to the MB and then through the riser to troubleshoot any potential HW issues with the card
or riser.