Error about credentials


Hello to everybody,
I have create a simple iOS app that uses the iOS Social Framework (with SLRequest and only-native accounts) to tweet some messages. I works okey for most of my users, but some of the get the following error while posting the tweet (using SLRequest with API v1.1)

" Your credentials do not allow access to this resource. "

How can this be possible in an iOS with native iOS accounts? Is there anything I am missing?
My code for posting looks like:

      NSURL* url = [NSURL URLWithString:@""];
        NSMutableDictionary *params;
        params = [NSDictionary dictionaryWithObjectsAndKeys:status, @"status",nil];
        request =  [SLRequest requestForServiceType:SLServiceTypeTwitter
        [request setAccount:twitterAccount];  // A valid twitter account got from an ACAccountStore
       dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0ul);
       dispatch_async(queue, ^{
            [request performRequestWithHandler: ^(NSData *responseData, NSHTTPURLResponse *urlResponse, NSError *error) {

Is it possible to add an to add a twitter app consumer key/secret to the request?

Thanks in advance,