The outputs 10,11,12,13 are wired fine
Yes I was trying to say check they are defined as outputs in the software.
I did a similar project but with a different analogue multiplexer:-
http://www.thebox.myzen.co.uk/Hardware/MIDI_Footsteps.html
and needed no pull down resistors, nor specific delay after switching the multiplexors. I have run this with less than the maximum number of sensors plugged in without problems. However, it was actually measuring resistance rather than a simple voltage as so in effect has a 100K pull up resistor.