www return empty text on android

I have this code:

 public tk2dTextMesh debugMessage;
  
 IEnumerator CoExecute()
 {
     string data = "some data";
     WWW req = new WWW(serverName + "/?" + data);
     yield return req;
  
     if (!string.IsNullOrEmpty(req.text))
        debugMessage.text = req.text;
     else
        debugMessage.text = "No result??";
  
     result = JsonMapper.ToObject<MyClass>(req.text);
     if (!string.IsNullOrEmpty(result.error))
     {
        debugMessage.text = result.error;
     }
 }

Note: debugMessage is a text mesh from 2DToolkit asset. It works fine in the editor but after building on android the debugMessage shows "No result??" which means that the text returned in the request is either null or empty and it's not an error in the request since debugMessage didn't show error.

Here's my AndroidManifest file so please check it and tell me what's wrong.

 <?xml version="1.0" encoding="utf-8"?>
 <manifest xmlns:android="http://schemas.android.com/apk/res/android" package="com.company.project" android:theme="@android:style/Theme.NoTitleBar" android:versionName="0.8.8" android:versionCode="1" android:installLocation="preferExternal">
   <supports-screens android:smallScreens="true" android:normalScreens="true" android:largeScreens="true" android:xlargeScreens="true" android:anyDensity="true" />
   <application android:icon="@drawable/app_icon" android:label="@string/app_name" android:debuggable="false">
     <activity android:name="com.unity3d.player.UnityPlayerNativeActivity" android:launchMode="singleTask" android:label="@string/app_name" android:configChanges="fontScale|keyboard|keyboardHidden|locale|mnc|mcc|navigation|orientation|screenLayout|screenSize|smallestScreenSize|uiMode|touchscreen" android:screenOrientation="sensorLandscape">
       <intent-filter>
         <action android:name="android.intent.action.MAIN" />
         <category android:name="android.intent.category.LAUNCHER" />
       </intent-filter>
       <meta-data android:name="unityplayer.ForwardNativeEventsToDalvik" android:value="false" />
     </activity>
   </application>
   <uses-sdk android:minSdkVersion="9" android:targetSdkVersion="17" />
   <uses-feature android:glEsVersion="0x00020000" />
   <uses-permission android:name="android.permission.INTERNET" />
   <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
   <uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
   <uses-feature android:name="android.hardware.touchscreen" />
   <uses-feature android:name="android.hardware.touchscreen.multitouch" android:required="false" />
   <uses-feature android:name="android.hardware.touchscreen.multitouch.distinct" android:required="false" />
 </manifest>