Using CD4021 Daisy Chain (not matrix) to Send noteOn and noteOff Messages

I have set my circuit up exactly like it is set up here...

Oh dear, their are no decoupling capacitors on that circuit. These are needed for reliable operation, they are not optional. You need a 0.1uF ceramic capacitor between the power and ground pin of each shift register.

http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html