Perhaps he's a bit shy or hesitant to respond to your text directly. By watching your story, he can keep a bit of distance while still being involved. It might also be that he was in a hurry when he saw your text and only had time to quickly view your story.
There are several possibilities. She might be a bit shy or hesitant to reply to your text directly. Watching your story could be her way of showing some level of interest without fully engaging in a conversation. Or perhaps she was in a situation where she couldn't text back immediately, like being in a meeting or with family, but had a moment to quickly look at your story. Another reason could be that she doesn't really want to have a deep text conversation but is casually interested in your updates as shown by watching your story.
Perhaps he has some sort of issue with you, although it's not very likely just based on this one instance. But if there has been some tension or misunderstanding between you two recently, it might have influenced his decision not to watch your story. For example, if you had an argument and he's still a bit miffed, he might be avoiding any form of engagement with your content for the time being.
Well, it might be that they're trying to keep their distance while still being a bit nosy. They didn't wish you a happy birthday because they don't want to give you the wrong idea that they want to be back in your life in a big way. But watching your story shows that they're still interested in what's going on with you on some level.
There's a chance he's not interested in the content you usually post. If your stories are mainly about things he doesn't care for, like a particular type of fashion that he has no interest in, he might skip it.