Rest integration - not returning values

I am trying to integrate with another automation system using the Rest integration. I have validated the yaml, but the sensor isn’t updating but there are no errors in the log. How can I find out if it is polling and what it is seeing returned?
config.yaml:

sensor:
  - platform: rest
    resource: http://192.168.1.137/api/v1/Ports.json/Thyone%20Temperature/0
    method: GET
    name: Outside Temp
    device_class: temperature
    value_template: '{{ value_json["CortexAPI"]["PortEvent"]["Value"] }}'
    unit_of_measurement: 'C'
    scan_interval: 13
  - platform: rest
    resource: http://192.168.1.137/api/v1/Ports.json/Bed%20Temperature/0
    method: GET
    name: Bed Temp
    device_class: temperature
    value_template: '{{ value_json.CortexAPI.PortEvent.Value | round(1)  }}'
    unit_of_measurement: 'C'
    scan_interval: 11

data returned from the resource (visible in a browser)

{
  "CortexAPI":{
    "PortEvent":{
      "FriendlyName":"Outside Temperature",
      "IDNumber":"51",
      "PortNumber":"0",
      "PortDescription":"Temperature Output",
      "Type":"ValueOut",
      "Value":"13.08984375",
      "State":"false",
      "NoticeText":"13.09"
    }
  }
}

this is what it looks like in the UI
image

Have you tried it without the value_template, to see what is returned?

Also are there any warning or error messages? You can also enable debug output with:

logger:
  default: info
  logs:
    homeassistant.components.rest.sensor: debug

These debug messages go to home-assistant.log.

I removed the value template and it was an authentication problem. I’d removed authentication from a specific IP address, but the hassbian has 2 interfaces and I’d fixed these to different address, D’Oh.
Thanks for the debug tip, I’ve a feeling it will be useful!

So it is now working with authentication removed, and a longer interval, next step to re apply basic authentication.

Huge thanks, I was utterly stuck.