Pushing Docker Container to ECR Hangs Then Fails

After running the command

docker push 123456.dkr.ecr.us-west-2.amazonaws.com/myapp

I then see

111111 Retrying
222222 Retrying
...
444444 Waiting
...
EOF

I know that I authenticated to AWS through the AWS CLI. What could be blocking my attempts to push my Docker image to ECR?