Hmm, I've got a core 16, but they use the same matrix truck. - I've had the wheel off already due to a puncture, but didn't have any spacers... - The hub should split in two with a bearing in each side.
The trucks contain nylon lock nuts, so there's no need to overtighten. Saying that, no matter how hard you do up the bolt, the wheels should spin due to the nature of the bearing.
Do all the wheels wobble? Does the wheel move side to side with free play between the axel shaft and bolt? - if so then there's a spacing problem. Or does the wheel sit snuggly, but the wheels wobble? - This could be a fault with the hub, i.e. bearing has been pushed out of it's seat, not sitting at 90 degrees to the axel.