Cases usually come with a generic i/o shield that hardly ever fits.
A new, out of the box motherboard always comes with one that fits.
If you pop out the one that came with the case and can properly fit the motherboard without any shield present, you have found your problem.
As long as nothing is shorted, this is a physical problem, not an electrical one.
Look at the bottom of Step 2:http://www.tomshardware.co.uk/forum/274745-13-step-step-guide-building