I have a boy who is exactly the same way!
He lives right next to a female, and he flirts with her constantly, showing every iota of fin he's got (which is a lot, but it's a nearly transparent brown) and dancing back and forth in front of her. But he's never flared at her.
When I had him in a divided tank with a male plakat, he cowered instead of flared back.
But the female next to him, who's also a coward, seems to like him back. Which almost tempts me to toss them together and see what happens, but neither is suitable breeding stock, they were both rescues.