Check you EC2 instance quotas to ensure you are able to launch VT1 instances - should be named "Running On-Demand G and VT instances" or "All G and VT Spot Instance Requests" for Spot instances.

Check that your job memory requirements leave some room for the ECS agent. Batch reserves a small portion of memory in the instance for this and other system tasks.

Check the job status. Blocked job queue events should update the job status if there is something wrong with the request.