UICollectionView not consistent layout, UICollectionViewFlowLayout

I am having trouble displaying the cells in a UICollectionView in a consistent manner. The initial display of the cells is correct, however every time the user scrolls past and then back to a group of cells the display is incorrect. Rows should only ever contain 2 or 1 cell(s). 2 cells each taking half the display width, 1 cell taking full width.

- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath { return [self preferredSizeForIndexPath:indexPath]; } - (CGSize)preferredSizeForIndexPath:(NSIndexPath *)indexPath { BOOL isLastObjectInSection = NO; NSString *sectionKey = [[arrCollectionData[indexPath.section] allKeys] objectAtIndex:0]; DLog(@"SectionKey: %@", sectionKey); NSArray *arrSection = [arrCollectionData[indexPath.section] objectForKey:sectionKey]; DLog(@"ArrSection: %@", arrSection); if ( arrSection[indexPath.row] == arrSection.lastObject ) { if( arrSection.count % 2 != 0 ) { isLastObjectInSection = YES; } } CGSize cellSize = CGSizeZero; if (UIDeviceOrientationIsLandscape([[UIDevice currentDevice] orientation])) { if (isLastObjectInSection == YES) { cellSize = CGSizeMake(IPAD_BADGE_WIDTH_LANDSCAPE_WIDE, IPAD_BADGE_HEIGHT_LANDSCAPE_WIDE); } else { cellSize = CGSizeMake(IPAD_BADGE_WIDTH_LANDSCAPE, IPAD_BADGE_HEIGHT_LANDSCAPE); } } else { if (isLastObjectInSection == YES) { cellSize = CGSizeMake(IPAD_BADGE_WIDTH_WIDE, IPAD_BADGE_HEIGHT_WIDE); } else { cellSize = CGSizeMake(IPAD_BADGE_WIDTH, IPAD_BADGE_HEIGHT); } } DLog(@"CellSize: %@", NSStringFromCGSize(cellSize)); return cellSize; } 

Below is a sample of the collection data.

Printing description of self->arrCollectionData: <__NSArrayI 0x94bbc40>( { "March 12, 2013" = ( "<FMLeafTimelineContainer: 0x94b2430>", "<FMLeafTimelineContainer: 0x94b3670>" ); }, { "February 25, 2013" = ( "<FMLeafTimelineContainer: 0x94b4500>" ); }, { "February 14, 2013" = ( "<FMLeafTimelineContainer: 0x94b48f0>", "<FMLeafTimelineContainer: 0x94b3a60>" ); }, { "February 12, 2013" = ( "<FMLeafTimelineContainer: 0x94b3ce0>", "<FMLeafTimelineContainer: 0x94b2b00>" ); }, { "February 4, 2013" = ( "<FMCommunityTimelineContainer: 0x94b4e90>", "<FMCommunityTimelineContainer: 0x94b5050>", "<FMCommunityTimelineContainer: 0x94b5f70>" ); }, { "January 30, 2013" = ( "<FMCommunityTimelineContainer: 0x94b6ad0>", "<FMCommunityTimelineContainer: 0x94b5a90>" ); }, { "January 24, 2013" = ( "<FMCommunityTimelineContainer: 0x94b5d00>", "<FMCommunityTimelineContainer: 0x94b6d90>" ); }, { "January 22, 2013" = ( "<FMCommunityTimelineContainer: 0x94b6440>" ); }, { "January 21, 2013" = ( "<FMCommunityTimelineContainer: 0x94b6260>", "<FMCommunityTimelineContainer: 0x94b62e0>", "<FMCommunityTimelineContainer: 0x94b70c0>", "<FMCommunityTimelineContainer: 0x94b55a0>", "<FMCommunityTimelineContainer: 0x94b82d0>", "<FMCommunityTimelineContainer: 0x94b78b0>" ); }, { "December 20, 2012" = ( "<FMCommunityTimelineContainer: 0x94b53f0>" ); }, { "December 6, 2012" = ( "<FMCommunityTimelineContainer: 0x94b7200>" ); }, { "December 4, 2012" = ( "<FMCommunityTimelineContainer: 0x94b72b0>" ); }, { "November 19, 2012" = ( "<FMCommunityTimelineContainer: 0x94b7ae0>" ); } ) 

The pics below all demonstrate the same section of the collection view.