Placement Groups Capacity
When using the cluster placement group type, you can encounter insufficient capacity errors. To avoid such a setback consider the following suggestions.
Single Launch Request
It is recommended that you launch the number of instances that you need in the placement group in a single launch request, and that you use the same instance type for all instances in the placement group. If you try to add more instances to the placement group later, or if you try to launch more than one instance type in the placement group, you increase your chances of getting an insufficient capacity error.
Start. Stop. Relaunch.
If you receive a capacity error when launching an instance in a placement group that already has running instances, stop and start all of the instances in the placement group, and try the launch again. Restarting the instances may migrate them to hardware that has the capacity for all the requested instances.
Please refer to the following link for more on placement groups and their rules and limitations.