Three things to check:
2. When you unlink from Dropbox, does the folder that won't sync stay in the list? If so, then swipe on its name and then tap delete while unlinked. Then re-link to Dropbox. It should re-sync.
3. For the folder that won't sync, is it colored red? If so, then you'll need to delete it from the app and then re-sync (see step 2).
Without knowing more, I'd recommend simply deleting and reinstalling the app from the App Store. That should clear out any problems in PlainText itself; if the folder still won't sync, then please check the folder for Dropbox errors.