I think Lewis was doing more character development in Caspian, and Susan's fearful character was starting to show through. If you read the story, you see that she's always the timid one, hanging back, criticizing, looking on the dark side. She's the last one to see Aslan, and His first words to her are, "You have listened to your fears, child."
If the BBC version (which I've never seen) portrays Susan as one who never smiles, they were probably trying to capture that aspect of Susan. Not smiling at all is a bit severe - she does have some fun, and takes part in the romp with Aslan, Lucy, & the crowd, but her fear-driven character is definitely showing.