Explore EMR step hanging on RUNNING state

I'm not sure it was dataflow-runner issue (quite likely it wasn't), but want to explore it further.

I tried to run simple Spark job with following cluster specification:

{
  "schema": "iglu:com.snowplowanalytics.dataflowrunner/ClusterConfig/avro/1-1-0",
  "data": {
    "name": "dataflow-runner - snowflake transformer",
    "logUri": "s3://mylogs/logs/",
    "region": "us-east-1",
    "credentials": {
      "accessKeyId": "env",
      "secretAccessKey": "env"
    },
    "roles": {
      "jobflow": "EMR_EC2_DefaultRole",
      "service": "EMR_DefaultRole"
    },
    "ec2": {
      "amiVersion": "5.5.0",
      "keyName": "mykey",
      "location": {
        "classic": {
          "availabilityZone": "us-east-1a"
        }
      },
      "instances": {
        "master": {
          "type": "m1.medium"
        },
        "core": {
          "type": "m1.medium",
          "count": 1
        },
        "task": {
          "type": "m1.medium",
          "count": 0,
          "bid": "0.015"
        }
      }
    },
    "tags": [ ],
    "bootstrapActionConfigs": [ ],
"configurations": [
      {
        "classification": "core-site",
        "properties": {
          "Io.file.buffer.size": "65536"
        }
      },
      {
        "classification": "mapred-site",
        "properties": {
          "Mapreduce.user.classpath.first": "true"
        }
      }
    ],
    "applications": [ "Hadoop", "Spark" ]
  }
}

However, no matter what I submitted - job hung in RUNNING state, not producing any output. To make it work I made two changes:

1. Changed location to {"vpc": null}
2. Bumped core and master to m2.xlarge

Not sure what from above worked and whether it was EMR misconfiguration or something from dataflow-runner, but maybe it's worth for raising.

Start by reproducing the submitted EMR cluster configuration and compare it with the working changes: VPC location and m2.xlarge master/core instances. Determine whether dataflow-runner or EMR configuration causes the job to remain in RUNNING without output; no files or tests are named in the issue.