Why some emails cannot be set as reminders?

I have this issue in my macbook... some emails cannot be set as reminders, however it does not happen with all my emails. For some emails, I drag the message into the reminder window, but i cannot put it in there. For some other, I drag the message and appers a green cross and the app let me put the email as a reminder.

I have not realized yet why? Can anybody give some advice?

Thanks in advance,

Andrés