uiscrollview 不水平滚动

我下面的 UIScrollView 没有水平滚动，请在这里帮助我..

FirstView = [[UIView alloc] initWithFrame:CGRectMake(60, 0, 100, 150)];
    [FirstView setBackgroundColor:[UIColor clearColor]];

    SecondView = [[UIView alloc] initWithFrame:CGRectMake(320+60, 0, 100, 150)];
    [SecondView setBackgroundColor:[UIColor clearColor]];

    HolderView = [[UIView alloc]initWithFrame:CGRectMake(0, 0, 1000, 150)];

    scrHorizontalScroll = [[UIScrollView alloc]initWithFrame:HolderView.frame];
        [scrHorizontalScroll setBackgroundColor:[UIColor redColor]];
    [scrHorizontalScroll setContentSize:CGSizeMake(999, 150)];
    [scrHorizontalScroll setScrollEnabled:YES];
    [HolderView addSubview:scrHorizontalScroll];
[scrHorizontalScroll addSubView:FirstView]; [scrHorizontalScroll addSubView:SecondView]; [HolderView addSubView: scrHorizontalScroll];

HolderView 没有水平滚动，但发生了这种情况，请帮忙

注意：我有一个 mainView，它根据框架坐标在其顶部添加此 HolderView。

[mainView addSubView:label].... [mainView addSubView:HolderView];

这是完整的结构

滚动视图仅在其内容大小为bigger比它的框架。