It sounds like you've already taken several troubleshooting steps to try to resolve the issue with updating transactions in your Amazon Business app. Thank you for your patience and effort, PinkyK.
Sometimes issues can be on Amazon's end due to system maintenance or outages. You can visit the Amazon Business status page or their official social media channels for any announcements regarding ongoing issues that could be impacting the app’s functionality.
If no visible issues have been found on the website, I recommend reaching out to their support team for further assistance. They can perform a more in-depth investigation to determine why this issue is occurring and help you resolve it efficiently.
To give you more information on how bank transaction updates, check out this article: Use Automatic and Manual Online Banking Updates.
Additionally, once everything is already working fine, QuickBooks suggests categories based on how you categorize similar transactions. I've added this article for reference: Categorize Online Bank Transactions in QuickBooks Online.
If there's anything else and specific that I can do for you, or any other inquiries when updating transactions, please feel free to reach out. I'm here to ensure your concerns are addressed in the most effective way. Have a wonderful day ahead filled with success.