You probably use V3 scheduler. You have Maximum reviews a day set to 50 and you have 50 (or more) cards to review. It means your daily limit is full therefore no new cards are introduced. New cards will be introduced when you have less than 50 card for review.
Yes, I use the V3-scheduler. And there are hundreds of due cards in the deck.
Maybe it is my fault: I am afraid I don’t understand the Just_Due and Over-Due deck issue in the manual.