Hi Amanda,
Opens are really tricky. They only register when an actual download of a Pixel happens. For example, if someone is previewing the email, it wont trigger. It is possible that you are doing a test sampel versus, selecting al ead and doing a flow action against it, but functionality aside, I would recommend a stronger trigger than an open.
To fix your trigger:
1. Try an actual flow action against the lead, then open it by double clicking and downloading images which might be in the email
2. Make sure its not just a text email etc.
Hope this helps!
M