This has been an on-going problem for me too, but I found a fix that I hope helps everyone:

1. Go to Products & Services - Click Edit to the right of the item that you're looking for.

2. Change the Income account to a different account than what's showing and Save.

3. Go back to the invoice and check to see if the item is now in the dropdown, but don't choose it yet.

NOTE: You might have to exit the invoice first then recreate if new or reselect if it's an existing one.

4. Return to the item in Products & Services and change the account back to the correct one.

5. Go back to the invoice and choose the item in the drop down box. Hopefully it's still there.