no front back sound difference

Hi,
I'm developing an app for Android using OpenAL for 3D sound. The app 
works but I hear the same sound for front/back relative source 
positions. Moreover, I can't use a Lowpass filter; I can create it, but 
it doesn't works if I enable it. Why?

I develop apps in Java, using jni for OpenAL library. I use a google 
tablet Nexus 7.

Thanks in advance for your responses
Dario
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