Paragraph tag won't apply

Hi Everyone,

I'm encountering a really frustrating issue, maybe you guys can help me out (note, I am a beginner user of FrameMaker). Ocassionally, when I highlight text > Apply > select a new Paragaph Tag, the tag won't adhere to the text. When I close FrameMaker out completely and try again, the tag then adheres to the text. Am I doing something wrong? Does this sound like a bug? Am I too low on memory?

Thanks!