4 to 16 decoder made by two 3 to 8 decoders not working properly
With what you have in the truth table and what you have wired in the output (right) side of the 3:8 decoders, the input (left) side of the decoders should look like this:


  y --> A2
  
  z --> A1
  
  w --> A0


You mixed the LSB of input with MSB.