UICollectionReusableView method not being called

It seems that you have to give your header a non-zero size or collectionView:viewForSupplementaryElementOfKind:atIndexPath isn’t called. So either set the headerReferenceSize property of the flow layout like so:

flowLayout.headerReferenceSize = CGSizeMake(self.collectionView.frame.size.width, 100.f);

Swift 5+

flowLayout.headerReferenceSize = CGSize(CGSize(width: self.collectionView.frame.size.width, height: 100))

or, implement collectionView:layout:referenceSizeForHeaderInSection if you want to vary the size by section.