Why does the control button disable on lock screen and command center when playing Music?

Question

Why dos the control button disable on lock screen and command center when playing Music? I use MPRemoteCommandCenter to control the access via remote. Please help me，tx very much.

Btw

1. The app was not killed.
2. I add admob SDK as my app Interstitial AD.

control button disable on command center

control button disable on lock screen

    if ([[[UIDevice currentDevice] systemVersion] floatValue] >= 7.1) {
    MPRemoteCommandCenter *commandCenter = [MPRemoteCommandCenter sharedCommandCenter];
    commandCenter.playCommand.enabled = YES;
    [commandCenter.playCommand addTarget:self action:@selector(playAction:)];

    [commandCenter.pauseCommand addTarget:self action:@selector(pauseAction:)];

    commandCenter.previousTrackCommand.enabled = YES;
    [commandCenter.previousTrackCommand addTarget:self action:@selector(previousTrackAction:)];

    commandCenter.nextTrackCommand.enabled = YES;
    [commandCenter.nextTrackCommand addTarget:self action:@selector(nextTrackAction:)];

    commandCenter.togglePlayPauseCommand.enabled = YES;
    [commandCenter.togglePlayPauseCommand addTarget:self action:@selector(playOrPauseAction:)];
}