You wrote this IAM policy to allow read access to a DynamoDB table but users still get AccessDenied errors. What is the likely problem?

You wrote this IAM policy to allow read access to a DynamoDB table but users still get AccessDenied errors. What is the likely problem?

{
  "Version": "2012-10-17",
  "Statement": [{
    "Effect": "Allow",
    "Action": "dynamodb:GetItem",
    "Resource": "arn:aws:dynamodb:us-east-1:123456789012:table/MyTable"
  }]
}