Suspect PCIe slot failure

I have a Targa computer for repair which was giving no signal to the screen from the Pciex card, if I connected to the onboard adapter everything worked fine.
I replaced the card with a new one but the new card is not recognised by the pc and no signal to the screen, I tried a third card but the same applies. If I connected the monitor and booted up with either card in it would boot into windows but the new card is not detected.
I am beginning to think that the PCIex slot has failed which means a new mobo, the mobo is an ASUS IPN73-BA.
Do you thonk that the slot has failed or should I look at something else before I condemn the mobo.