I appreciate you doing everything you could to fix the issue. @summerimex. Don't worry, I know some troubleshooting steps to help you fix the Quantity discount from not populating.

The issue that you've encountered is an indication of potential data integrity. Usually, this can be caused by internal (within QuickBooks) or external (file location) factors. To rectify this, let's run the Verify and Rebuild Data tool. The Verify Data will identify any data issues within your company file. Then, the Rebuild tool will self-resolve it. 

To verify data:

1. Open the company file.
2. Click the File tab at the top menu.
3. Select Utilities.
4. Choose Verify Data.
   
   
    

If it detects an error, proceed and Rebuild your data:
 

1. Follow Steps 1-3 above, then click Rebuild Data.
2. Choose OK if you receive a message to back up your company file.
3. After creating the backup file, the Rebuild Data utility will run automatically.
4. Tap OK when you get the message Rebuild has completed.
5. Go to File, select Utilities then select Verify Data again to check for additional damage.

For more details about the process, see this article: Verify and Rebuild Data in QuickBooks Desktop.

I'm adding this guide to learn how to record in case you receive advance payments on sales orders from your customer: Receive customer prepayments on sales orders in QuickBooks Desktop Enterprise.

Please let me know how else I can help you with managing Quantity discounts in QuickBooks. You can leave your reply below and I'll provide the necessary help.