How to optimize a process parameters using ANN and GA?

I have 5 parameters and each in 3 levels like below:

x1=[5 8 10]

x2=[5 8 10]

x3=[1 1.5 2]

x4=[0.216 0.191 0.224]

x5=[1 2 3]

I extracted a function(net) from ANN for predicting the output resulted from setting these parameters.

I want to find the minimum possible output using GA but it doesn't give the exact answer and I don't know why!

Attached you can find my codes. Could anybody please help me with this?