Hello Perry
The one that is 'identical' - has it been updated to 6.5? Whether it is pre-upgrade or not, compare the driver and firmware in-use on both:
Log into the iDRAC and check which firmware each is using - as I said above, potentially they are not being picked up due to driver/firmware mismatch. Should be able to get part number and/or VID/DID/SVID/SSID there also which will allow you to confirm which driver family and version and firmware should be used e.g.:
VMware Compatibility Guide - I/O Device Search
Do note that there are generally different HCL entries and driver/firmware recommendations depending on what the card is being used for (e.g. FC vs Network) but then again I can only see 'Network' usage references for those Broadcom 5719 on the HCL.
If it appears to have correct driver and firmware installed then you could always try rolling it back to previous working configuration to confirm there is no physical issue with the card and/or look in the boot log if they are being detected at all.
Bob