Can't get newly built computer to display on monitor

I have been having a lot of trouble trying to get a computer I put together to show any video. I have tried three different motherboards and I have the same problem with all three, but If i try the components on a computer that has worked in the past, they work fine, I have replaced the ram with different chips, used three different processors and three different video cards, I am totally stumped. The monitor works fine on other computers so I know the monitor and cables are ok. I have also tried a few different power supplies and still no luck. I tried to reset the cmos and take out the battery but it didn't work either. I can't understand why these three mobos don't show any video but another one I have works fine.

Here are the mobos that won't work:

MSI 865PE Neo2-p
DFI 848P-AL
ABIT BE711

The board that I tried that works is a Soyo Dragon p4x400 lite

could it be a problem with the voltage of the AGP slot, the cards I have been using are 2X/4X AGP cards. I haven't tried a PCI card.

do the 3 boards that won't work have onboard video? if they do, then you need to go into the BIOS and change it over from one source to the other.