Being that they are remote collectors, I'll assume they are on a different network segment and there are likely firewalls between the RC and the data node. Make sure these ports are open.
vRealize Operations Manager 6.x 1235 TCP vRealize Operation Manager 6.0 nodes vRealize Operation Manager 6.0 nodes
Used by all nodes in the cluster to transmit resource data and key-value data for the Global xDB database instance.
Also make sure forward and reverse DNS are working properly.
There is a KB article saying that there are issues when trying to add multiple nodes in parallel. Might be worth following the KB article below:
vRealize Operations Manager Collector nodes are not considered cluster nodes, thus do not participate in intra-cluster communication. Ports 6061, 10000, & 443 are the ports required for communication between the vROps Manager cluster nodes and the remote collector.