Check in Admin -> Networks to make sure the network ranges match your IP structure, you do not have duplicated network IP ranges defined on one Data Collector or on two different Data Collectors. Check all ranges, including inactive ranges and inactive Data Collectors.

1) If the IP structure of your office changes, you will need to make sure the IP Ranges setup in the data collector reflect the new IP structure.

Example:

Old IP structure: 192.168.220.X
New IP structure: 10.0.0.X

This means, any of the old ranges in the data collector will need to be changed from the 192.168.220.X structure to the new 10.0.0.X structure. Otherwise, devices will/can not be collected from.

2) If the same network range is defined twice on a Data Collector it is possible the whole network range(s) will not be discovered or collected.

Example: 

Data Collector 1
Range 1: 192.168.1.20 to 192.168.1.30
Range 2: 192.168.1.1 to 192.168.1.254

In this example range 1 is included in range 2, so you either have to delete range 1 or edit range 2 so they are not overlapping each other.

3) If the same network range is defined twice on two different Data Collectors it is possible the whole network range(s) will not be discovered or collected.

Example: 

Data Collector 1
Range 1: 192.168.1.20 to 192.168.1.30

Data Collector 2
Range 2: 192.168.1.1 to 192.168.1.254

In this example range 1 on Data Collector 1 is included in range 2 on Data Collector 2.  You either have to delete range 1, or edit range 2, so they are not overlapping each other.

If one Data Collector can cover all network ranges then you should consider to only use one Data Collector on the account. Before removing a Data Collector, make sure all network ranges are defined on the Data Collector you are planning on using.