MSI-X resource become less and less

let's take Broadcom bnx2x as example:

first of all I can insmod bnx2x.ko multi_mode=1 num_queue=16
but when I do this:
rmmod bnx2x; insmod bnx2x.ko multi_mode=1 num_queue=8   ->OK 8 queues
rmmod bnx2x; insmod bnx2x.ko multi_mode=1 num_queue=4   ->OK 4 queues

...
finally I do this:
rmmod bnx2x; insmod bnx2x.ko multi_mode=1 num_queue=4

I found the driver can only get 1 or 2 queues! it doesn't come back to 4+ queues any longer!

MSI-X resource become less and less, any resource isn't free?

kernel:2.6.27.19
XEON 5500 16 core

thanks
jon