Fast swiping down cause flexibleHeader not take effect immediately

[Hashem]

when I scroll down slowly my custom flexible header smoothly comes down. but when I scroll down fast, flexible header stuck in its place! I don't see this effect when I scroll up fast.
here is the gif:

my workaround is override setProgress: in my custom flexible header and reposition imageView manually:

- (void)setProgress:(CGFloat)progress
{
    [super setProgress:progress];   
    if (progress<0) {
        self.imageView.y = navBarHeight;
    }
}