subscription manager shows "insufficient" at first attempt of content host registration

I am running Satellite 6 and found strange issue while registering ESX-VMs to the satellite/Capsule Server via Activation key.

I have configured Virtwho to assign VCD subscriptions to VMs running on ESX hypervisors.

This is what happening ....

Installed Product Current Status:
Product Name: Red Hat Enterprise Linux Server
Status: Not Subscribed

                  Unable to find available subscriptions for all your installed products

+-------------------------------------------+
System Status Details
+-------------------------------------------+
Overall Status: Insufficient

Red Hat Enterprise Linux for Virtual Datacenters, Premium:
- Guest has not been reported on any host and is using a temporary unmapped guest subscription.

So it does not get register at first attempt , in second attempt i do "subscription-manager attach --auto". After giving this command it finds the subscription and gets registered.

I have checked,
- VCD subscription assigned to ESX host
- ESX VCD subscription added to Activation key
- no err in /var/log/rhsm
-virtwho is running without error
- virthwho config able to find ESX hosts based on config

How its failing at first attempt ?