You created an automation, it sent you an email. You want to track the open and send a new email to not-openers. Great!
The most common issue is forgeting to put a WAIT period before CONDITIONs.
Let's check this automation:
After somebody gets the tag: "test" this automation will send the email and check the condition.
The problem is, that this process - sending this email and checking the condition - takes 0.0001 second. This is not enough time for your subscriber to open this email so everybody ends up on the NO branch.
What you should do:
ALWAYS use a WAIT period AFTER EVERY EMAIL SENDING:
That's it, you are ready to track who opened your email and who didn't and use it as a condition.