Please update your copy of Notes Keeper to v0.6.4.1610 to fix a bug.
The Bug: it affect the notes that are in a folder that its name contains more than one word. when you open the note it, Notes Keeper by mistake change something in the database and make it invisible in the folder and you just need to edit the database with a SQLite database manager like Firefox addon by searching at the folder column any word has space in it just edit it and remove the space.
e.g. new folder must be newfolder
that is it, now everything will become as normal.
v0.6.4.1610 had 4 new features
1 - Add to favorites.
2 - change text case
3 - note statistics on the status bar and you can click a button to have them in a copyable small window.
4 - Snap the main window of notes keeper to any edge you want in the screen.
any it has code fixed.
stable version will be released soon.