HI Kiran,
Try to do some troubleshooting.
1. Try to ping and traceroute the nodes that orion says "is down" from the Polling server (orion server) where that node is assigned. and then vice versa
2. Check for "possible" IP address conflict of nodes
3. If you have admin access, try to choose the nodes then "Poll Now" or "Rediscover"
4. If you don't need historical data of that nodes, you can try to delete and add back the node.