Observed IP range is generated based on the incoming broadcast packets it detects. Therefore, for NICs that see only unicast/multicast traffic, it displays None.
This can be an expected behavior if some vmnics are not receiving broadcast traffic from the switch. To check that the vmnics are load balancing, verify that there is traffic going over all of the NICs using the esxtop command (when running, enter n to select network statistics). You can also do this at the physical switch level with the counters provided by your vendor.
If every link sees a fair share of frames/packets, your form of load balancing/NIC teaming is probably only sending broadcast traffic over the link which is displaying information for Observed IP range.
This is not considered an issue and can be expected behavior- inquire with the physical switch vendor about broadcast traffic coming from the physical switch.