Chord problems on Musical Typing

Hello, I've got Garage Band '11 Version 6.0.5 (428.5) and I've been having problems with Musical Typing. Many chords of three notes or more won't play. For example, BDF (played by hitting the J, L and ' keys) will not work. The pairs within it will work, but when those three notes come together, it's like they cancel out. That happens with almost every chord made up of three notes all within a semitone of each other, eg C, C# and D (played A W S). Is there any solution for this problem?

(My MacBook Pro runs OS X 10.9.2.)