New DC added but only picks up a few devices after rebalancing.

Document ID : KB000115095
Last Modified Date : 14/09/2018
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Upgraded CAPM to 3.5, then added a new DC (there was previously just a single DC).
Rebalancing caused only about 35 devices to move to the new DC.
Manual rebalancing has no effect.
Customer firewall team states that there are no FW-related impediments to the rebalancing.
Similarly, customer network team states that the devices currently on the original DC should all respond to the new DC.
CAPM 3.x
The new DC is unable to SNMP to any devices, and only could ping 35.
We tried
snmpget -v 2c -c <string> <target ip>
snmpget -v 2c -c <string> <target ip>

Works from old DC to all tested targets
Fails on new DC to all tested targets
Likely acl issue, may be firewall.