There is a know bug on Memonote Mac 1.2.1 for sync with dropbox. Please upgrade to Memonote 1.2.2 and do the following stop if you got sync problem.
Step 1, Stop sync on all device (Both Mac and iPhone). Step 2, Move ~/Dropbox/Notes.memonote to another place(for backup) or delete it. Step 3: Start dropbox sync on Mac, and start sync on other device 5-10 minutes later.