Couple of different designs out there. Pictures would help for a solution on this one.
Common issue:
I/O backplane has been installed from the outside or inside when it should be the opposite.
Shielded foam: constant pressure needs to be applied while the first few screws are place.
Over i/o tabs: occasionally need to be slightly bent to produce a reasonable installation pressure
Over i/o tabs: occasionally get stuck in or on connectors resulting in a difficult installation