Anyone else unable to write invoices or go to customers anymore through app?

Let's perform troubleshooting steps so we can fix your invoice issue using the app, UCall.

One of the reasons why this causes odd behavior it is because you're low on space or running too many apps at once, spotty Wi-Fi connection or you're on an old version of the app that hasn't been updated. To resolve the problem, follow the steps outlined below: 

If you're using an iOS, follow the steps below:

1. Go to Menu and tap Help & Feedback.
2. Tap Refresh Data.
3. Click Refresh to confirm.

Otherwise, refer to this instruction for Android:

1. Go to Menu, then select More Options ⋮.
2. Tap Settings, and click Refresh Data.
3. Tap YES to confirm.

On the other, if the issue is still happening, uninstall and reinstall the application to ensure you'll get the most recent version of the application. 

Moreover, QuickBooks Online (QBO) lets you record sales transactions in two ways. Learn how you can record payments for a single and partial invoice. Read this article for more details: Record Invoice Payments in QuickBooks Online.

Reply below if you have any questions when creating invoices using the app. The Community is always here to assist your needs anytime.