Tower Plus QBT1250, PCIe Slot 3 not working

I am commissioning a PC for a customer and ran into a problem with PCIe Slot 3 not working. Right now I am trying to install a Startech dual network card (P/N=ST1000SPEXD4) and a Startech 8-way serial/RS232 card (P/N=PEX8S1050LP). I need all 3 slots in a working condition in the event I need a 3rd expansion card for the customer. I have been using Optiplex machines for over a decade now with no such issue, using Windows 7/10/11.

This occurs on both the PCs I have to hand so I do not think it is a faulty unit. I do have access to more PCs to try though just in case it turns out I have multiple PCs with the same fault.

I have gone into the BIOS and I can see that the PC detects the serial card in the slot, it is just not being passed through to Windows but I cannot find anything in BIOS or the manual about Slot 3 being disabled under certain conditions. I disabled all the SATA and NVME slots except NVME 0 (where the SSD was fitted) as a hail mary but that did nothing.

Combinations tried present the following conditions.

Combination 1 - Initial install as it allows the RS232 daughter board to occupy Slot 4 where there is no PCIe connector

• Slot 1 --> Empty --> Need this working in the event a 3rd card needs to be installed
• Slot 2 --> Dual network card --> Seen in Windows and works fine
• Slot 3 --> Serial/RS232 card  --> Not seen in Windows but seen in BIOS

Combination 2 - Check to see if the Serial/RS232 card is at fault, but fault stays on PCIe Slot 3

• Slot 1 --> Empty --> Need this working in the event a 3rd card needs to be installed
• Slot 2 --> Serial/RS232 card --> Seen in Windows and works fine
• Slot 3 --> Dual network card --> Not seen in Windows

Combination 3 - Check to see if Slot 1 + 2 work at the same time, and they do

• Slot 1 --> Serial/RS232 card --> Seen in Windows and works fine
• Slot 2 --> Dual network card --> Seen in Windows and works fine
• Slot 3 --> Empty --> Need this working in the event a 3rd card needs to be installed

Combination 4 - Check to see if Slot 1 + 3 work at the same time, Slot 3 still is faulty

• Slot 1 --> Dual network card --> Seen in Windows and works fine
• Slot 2 --> Empty --> Need this working in the event a 3rd card needs to be installed
• Slot 3 --> Serial/RS232 card  --> Not seen in Windows but seen in BIOS

Now I am out of ideas outside of I have 2+ PCs with faulty PCIe Slot 3. Any suggestions before they are returned to the supplier as faulty?