PCI device can't be seen from PCI-X slot

Hi,

Here is my situation:

Intel Server Board: my PCI device can be recognized (can be shown on Device
manager) no matter which PCI or PCI-X slot the device is in.

Sun Prairie: my PCI device (same device) can be seen from Device Manager if
the device is in PCI slot, but won’t be seen if the device is in PCI-X slot.

Can anybody give me a hint?

Thanks,

zhongsheng