Please have a look !

I am using differential LNA , in input stages i have matching circuits, same at the output, but when i put BALUN to combine these Vin+ and Vin- and same procedure at the output ... then my S11, S22, S21, and S12 changes. I am using ADS tool..can't find out what is the problem.

can anyone help ?

Depends by balun ratio you can get impedance change from un-balanced to balanced ports.
Second thing, baluns usually have a limit for frequency range (up and down).

Check balun bandwidth