[Discuss-gnuradio] USRP failed to tune?

I have two USRP1 boxes and four RFX2400 daughter cards. It looks that one of the two USRP1 box does not recognize three of four daughter cards.
I pluged in the daughter card to a USRP1 one by one and ran benchmarck_tx.py script with the center frequency set to 2.4G. All four daughter cards ran ok on one chassis. However, one the other chassis, an error came out saying it failed to tune to 2.4G on three out of four cards.
If I insert one "accepted" card and one not accepted card on this bad USRP, the error message goes away.

Could you let me know if you have any clue?
Thanks,

Thomas