For me, you add your list criteria in the Smart List Tab (for instance email address is not empty, Lead Status is open etc)
The Random part is to be added to the Flow Tab.
Maybe you can do screenshot of your smart list and flow and I can tell you if I spot a mistake??
PS a common mistake is to leave AND in your smart list criteria. As a result, you can end up with impossible criteria like for instance Country = France AND Business Zone = NORAM