Hi, the spare boards might of been faulty unfortunately. However, if one works in one machine and not the other, that's very strange. Check they are pushed in the backplane properly, if it's not quite plugging in, that can cause problems.
If you have a board that won't boot in either machine and you have tried the obvious (different games etc) and its pretty much 'faulty' then you can try a trick, when the machine is turned on, gently put a bit of pressure with your finger and bend the board up, slowly, so it bows a bit, you might be lucky and then you will notice the red LED lights will start to run along as normal, keep your finger/pressure there. There is a good chance it will actually start booting and even be playable if you jam something there, but it won't last long as this is common of a fault with those boards, I think its the internal layers that have cracked and there is no way to repair this without spending more time than a replacement board is worth!