Dell 660s GT620 HDMI no signal

My Dell 660s comes with a GT620 low profile display card. There are DVI and HDMI output on the card. Previously I used the DVI to connect with the monitor and it was okay. Recently I changed to another monitor which does not have DVI but HDMI so I connected it with the HDMI on the card but there was no signal. Then I tried the onboard HDMI and it works! How come? So the GT620 is now bypassed and become useless. What seems to be the problem?