Ionic 4 PWA app - cache no longer working?

Not sure what I have done, my PWA was caching (or so I thought?).

I’ve added a couple of extra pages that draw data from an API and now when I disconnect from the internet these pages don’t display any data, no cached data (although the app still runs).

Have I forgotten to do something?

I am building with …

ionic build --prod --service-worker

ionic info
Ionic:
   Ionic CLI                     : 5.1.0 (C:\Users\User\AppData\Roaming\npm\node_modules\ionic)
   Ionic Framework               : @ionic/angular 4.11.1
   @angular-devkit/build-angular : 0.801.3
   @angular-devkit/schematics    : 8.1.3
   @angular/cli                  : 8.3.12
   @ionic/angular-toolkit        : 2.0.0

Utility:
   cordova-res : 0.6.0
   native-run  : 0.2.5

System:
   NodeJS : v10.16.0 (C:\Program Files\nodejs\node.exe)
   npm    : 6.9.0
   OS     : Windows 10