Thanks for posting here in the Community. I'm here to help make sure that all your vendors will show on the list when preparing 1099s in QuickBooks Online (QBO).
The transactions that contribute to a specific vendor meeting the $600 threshold will only apply if their respective accounts are selected. To isolate this, make sure that the expense account you use to track your vendor payments is selected on Step 2 - AccountsBox 7 on the 1099-MISC form.
1. Go back to Vendors menu again, and click Prepare 1099's.
2. Select Back until you're on Step 2 and verify that all of these accounts are selected on the drop-down menu of the Box 7.
Once done, continue the process and you should now see them on Step 4 - Review. For your reference, you may check out these article for further guidance:
That should do it! Please let me know how it goes by leaving a comment below. I'm only a post away if you have any follow-up questions about the process. Wishing you and your business continued success.