For iOS users (Apple devices), simply update the app to the latest version. This issue can occur on app versions older than 2.2.6. You can see the version of the app you're on under the settings icon in the app.
If the receipts don't automatically upload after the update, please cancel the uploads and retry as below. This can be required if your version of the app is particularly old I'm afraid.
Click into the receipt and click on 'Cancel upload':
Then Select 'Retry Upload':
Do please let us know if there's anything else we can help with.