I can send invoices from my Gmail account using QB on the Web, but nothing gets sent when I try to send from the app

I've got your back  in sending your invoices through mobile app , klywelyek-gmail-.
 

The mobile app might need to be refreshed so you can emailed invoices successfully. Issues in QuickBooks can be fixed by clearing your app's data. If you haven't tried the following steps, I suggest performing them.

For QuickBooks Online App:

Android

1. Go to Menu ☰ then tap More Options ⋮.
2. Tap Settings then Refresh Data.
3. Tap YES to confirm.

iOS

1. Go to Menu ☰ and tap Help & Feedback.
2. Tap Refresh Data.
3. Note: A message appears “Caution. If there was a problem getting your latest QuickBooks Online data, you can refresh it now. It may take a few minutes.”
4. Tap Refresh to confirm.

In your Phone Settings:

Android 

1. From your phone's settings, select Apps (varies by device).
2. Choose the QuickBooks app from the list and select Storage.
3. Select clear data.

iOS 

1. From your phone's settings, select Settings, then, click on General and choose iPhone Storage.
2. Select an app that's taking up a lot of space.
3. To free up the storage without deleting the documents and data for the app, select Offload App.  

If the issue persists, you can uninstall and then reinstall the app to refresh its setup.

If you want to add customization on your sales forms in the future, you can check this article for a guide: Customise invoices, estimates, and sales receipts in QuickBooks Online.

Let me know if you were able to get through. If you have any more questions, please don't hesitate to let me know. I will be around. Keep safe! and have a good one.