iOS 8 push notification action buttons - code in handleActionWithIdentifier does not always run when app is in background

I am adding two action buttons to my push notifications on iOS 8: an Accept button and a Deny button. Neither button will open the app, but different server requests will be made depending on which button is pressed. Here's my setup:

+ (void)requestForPushNotificationToken {
    UIApplication *application = [UIApplication sharedApplication];
    // if ios 8 or greater
    if ([application respondsToSelector:@selector(registerUserNotificationSettings:)]) {
        UIMutableUserNotificationAction *acceptAction = [[UIMutableUserNotificationAction alloc] init];
        [acceptAction setActivationMode:UIUserNotificationActivationModeBackground];
        [acceptAction setTitle:@"Accept"];
        [acceptAction setIdentifier:@"ACCEPT_ACTION"];
        [acceptAction setDestructive:NO];
        [acceptAction setAuthenticationRequired:NO];

        UIMutableUserNotificationAction *denyAction = [[UIMutableUserNotificationAction alloc] init];
        [denyAction setActivationMode:UIUserNotificationActivationModeBackground];
        [denyAction setTitle:@"Deny"];
        [denyAction setIdentifier:@"DENY_ACTION"];
        [denyAction setDestructive:NO];
        [denyAction setAuthenticationRequired:NO];

        UIMutableUserNotificationCategory *actionCategory = [[UIMutableUserNotificationCategory alloc] init];
        [actionCategory setIdentifier:@"ACTIONABLE"];
        [actionCategory setActions:@[acceptAction, denyAction]
                        forContext:UIUserNotificationActionContextDefault];

        NSSet *categories = [NSSet setWithObject:actionCategory];

        UIUserNotificationSettings *settings = [UIUserNotificationSettings settingsForTypes:(UIRemoteNotificationTypeBadge | UIRemoteNotificationTypeSound | UIRemoteNotificationTypeAlert) categories:categories];
        [application registerUserNotificationSettings:settings];
    } else if ([application respondsToSelector:@selector(registerForRemoteNotificationTypes:)]) { // ios 7 or lesser
        UIRemoteNotificationType myTypes = UIRemoteNotificationTypeBadge | UIRemoteNotificationTypeAlert | UIRemoteNotificationTypeSound;
        [application registerForRemoteNotificationTypes:myTypes];
    }
}

Then, in my delegate method, I am specifying actions to be taken when user pressed one of the action buttons:

- (void)application:(UIApplication *)application handleActionWithIdentifier:(NSString *)identifier forRemoteNotification:(NSDictionary *)userInfo completionHandler:(void (^)())completionHandler {
    if ([identifier isEqualToString:@"ACCEPT_ACTION"]) {
        // Sending a request to the server here
    }
    else if ([identifier isEqualToString:@"DENY_ACTION"]) {
        // Sending a request to the server here
    }

    if (completionHandler) {
        completionHandler();
    }
}

The ideal scenario is that the user does not need to launch the app in the whole process; pressing Accept or Deny will make different calls to the server. With the code above, I am seeing very unstable behaviors with the button actions:

• A lot of times, the code in the action handler doesn't execute when app is in background and no server calls are made at all; when this happens, if I tap on my app icon and launch my app, the handler code will immediately be run upon launching the app.
• Occasionally, the handler code gets triggered and everything works fine. Server requests are made as soon as I press one of the action buttons.
• If I put breakpoints in my Xcode and step through the handler code, the success rate is also 100%. I do not need to launch my app, and handler code gets executed when button is pressed.

Could anyone please help me figure out what's causing such unstable behavior? Thanks in advance.