Concourse S3 resource can't find simple file; 'no versions available'

I've got a Concourse trigger set to detect when a particular file appears in an S3 bucket. Using this resource: https://github.com/concourse/s3-resource . Configuration is like so:

- name: s3-trigger-file
  type: s3
  source:
      bucket: mybucket
      regexp: filename_that_doesnt_change
      access_key_id: {{s3-access-key-id}}
      secret_access_key: {{s3-secret-access-key}}

I use it as a trigger like so:

jobs:
  - name: job-waiting-for-s3-file-to-appear
    public: true
    plan:
    - get: s3-trigger-file
      trigger: true

Seems like an extremely simple configuration. However, when I start the job and put a file in the bucket, I get 'no versions available'.

Any suggestions on how I might proceed in troubleshooting? Thanks ~~

Concourse is not detecting s3-trigger-file. Here are a few potential causes:

• The access-key-id and secret-access-key you are using do not have access to the file.
• The filename in your regexp: is incorrect. Make sure it's an exact match that includes the file extension.
• There is some networking configuration preventing your Concourse from talking to S3. You can ensure this is not the case by fly hijacking into the check container and using the Amazon CLI to manually pull the file.