What's going wrong?
K-9 by default has no threading. As a feature request I suggest that we implement a Gmail style threading. If people aren't familiar with how this works then they should try out both the Gmail app and web interface. I suggest this because, especially on a small screen, I see this as a huge time saver as you don't have to go searching for the other emails in the thread, you can just scroll up.
What version of K-9 are you using?
Is your email account a POP account, Exchange Account or an IMAP account?