I am posting value but some values taking null? why

 NSArray *myStrings = [[NSArray alloc] initWithObjects:post, post1, nil];
    NSString *joinedString = [myStrings componentsJoinedByString:@"&"];
   NSLog(@"post:%@",joinedString);
    NSData *postData = [joinedString dataUsingEncoding:NSUTF8StringEncoding allowLossyConversion:YES];