Computer wont restart

I have this problem on both my computer and my brothers. When I hit restart or hit the reset button on the case my pc doesn't always boot. It sounds like its working but there is no display. It works if I shut it down (or just crash it) then turn it on again. I'm not really bothered but it does get a bit worrying :(

Can anyone think of a possible cause?

Thanks

Edit: Would have been much easier if I had said that the graphics card doesn't work on a warm boot, sorry for the longish thread ;). Come to think about it we both use Xcylo PSUs. Could it be that?

PSU would be my first thought, although could just be an incompatibility, and if it is, it could be worth getting in contact with xclio, they might send you a couple of new ones like enermax did for me this week :)

So I could actually make a profit out of this or would have to send them back? :D

I would think that you have to send the old one back first, but ring them and see. They might just tell you to knob off :)