Going through all my ISA video cards and I found that the old Dell LCD monitor I was using for testing would show that it was getting a signal but nothing was showing up on the screen.

Dis a bit of digging and found that LCD monitors use pins 12 and 15 to communicate back the card for the EDID information.

If those pins are connected and the video card is old enough to not have that functionality, you get nothing on screen.

The fix is to either break pin 12 out of your VGA cable or use a cheap (thinner) VGA cable that doesn't have those pins connected.