Adobe AIR iOS app doesn't run in background

I have an iOS app made with Adobe AIR. It doesn't run in background even though I have done the following things I can think of,

1.  NativeApplication.nativeApplication.executeInBackground = true;

This is the first thing I did. I had another app which runs in background without problem, and this is the only trick I know I used there.

2. Experimented with <renderMode>direct</renderMode>, <renderMode>gpu</renderMode>, and <renderMode>auto</renderMode>. None worked.

3. Added,

<key>UIBackgroundModes</key>

  <array>

  <string>audio</string>

  </array>

Didn't work either.

Whenever I switch the app to background, no matter how short the time is, it always starts from the beginning when switched back. The only exception is when I take a phone call, in which case it seems to continue from where it left off. Can someone please tell me what is going on? Is there still hope left for me? Thanks a bunch.

BTW, the same app packaged for Android has no problem running in the background, and I didn't do anything to enable it.