Flow not working as expected: "Preview Trigger Setup" indicates it should

For my campaign I have setup a flow that sends out an email to each person subscribing to a list called “FB Ad Leads (Seasonal Calendar)”, which is coming in via Facebook Leads, and Seasonal Calendar Leads, which is coming in via a pop-up on our website.

I am having the following situation:

• Both lists contain emails and have the column “Language” filled with information 
• Going to the “Preview Trigger Setup” it shows that those contacts are eligible for this flow 
• For new contacts coming in, no email is send out
• For existing contacts, when “Back Populating Flow Recipients” no emails are send out.

Any support on this matter would be very much appreciated.

Any idea what might cause this issue to arise?

Thanks a lot for your help