UICollectionView with ContentInset is not Scrolling all the Way Down

If you are using flow layout try to set _collectionView.collectionViewLayout.sectionInset.

Today, by chance I discovered the solution!

Select your view controller and uncheck the option "Adjust Scroll View Insets".

With this option unchecked, iOS does not automatically adjust your insets of the view (and probably its subviews), which caused the problems for me ... Uncheck it and configure your scroll insets like this programmatically:

- (void)configureInsetsOfCollectionView
{
    [_collectionView setContentInset: UIEdgeInsetsMake(self.navigationController.navigationBar.bounds.size.height + [UIApplication sharedApplication].statusBarFrame.size.height + DEFAULT_SPACING, 0.f, _keyboardHeight + _toolbar.bounds.size.height + DEFAULT_SPACING, 0.f)];
    [_collectionView setScrollIndicatorInsets:UIEdgeInsetsMake(self.navigationController.navigationBar.bounds.size.height + [UIApplication sharedApplication].statusBarFrame.size.height, 0.f, _keyboardHeight + _toolbar.bounds.size.height, 0.f)];
}