OPA1664: Signal is getting distorted at the fourth stage of the Op Amp, Sallen Key configuration

Hi,

I am using OPA1664, a four channel precision Op Amp for amplification and low pass filtering (Sallen Key filter ; VCC/2 as a reference to non inverting pin for audio). I have created the design in the WEBBENCH. I am giving input signal of 50mV (P-P), there is gain of 2 in every stage making it total 16. The output signal is coming perfectly in the first, second and third stage, but there is somewhat distortion in the last stage, I don't know why, maybe phase shift or stability issue.

Third stage has voltage output of 360mV but fourth stage has output of 240mV only, why there is attenuation in the fourth stage. 

Please help with any suggestions: 

Regards,

WEBBENCH DESIGN( below) detail webench file webench_design_report.pdf:

The negative input was not coming so modified the first Op Amp as to also allow audio signal to get amplified. Only first Op Amp is seen (below).

Output from real circuit 

1) Input signal from signal generator(50.4mV P-P):                                                                                                          

 2) Output from first stage (97mV P-P)  

3) Output from second stage( 190mV P-P)                                                                                                          

 4) Output from third stage( 360mV P-P)

  5) Output from fourth stage (240mV P-P) and it is distorted, Third stage has voltage output of 360mV but fourth stage has output of 240mV only, strange.