Subclassing UICollectionViewFlowLayout cause blank cells - ios

Question

Using UICollectionViewFlowLayout ,i have 2 columns- collection view that works great .
Problem have started when i needed a constant distance between cells at the same row, when they have a dynamic height .
So , i have subclass UICollectionViewFlowLayout as everybody do, but what it did, is to eliminate the next cells to be loaded to screen, so while scrolling i have blank spaces ,without cells, and than they are suddenly loaded in late . looks like
1 2
3 4
6
8
10
Than, 5-7-10 suddenly appears late. seems that the subclass change the delegate method that creates the cells, and its now not being called at time, to load the next cells .
Whats wrong with it ? i am struggling 3 days with no results.
The subclass method that suppose to make the spacing equal(and cause the problem)
- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath
{
UICollectionViewLayoutAttributes* currentItemAttributes = [super layoutAttributesForItemAtIndexPath:indexPath];
if (indexPath.item < numColumns)
{
CGRect f = currentItemAttributes.frame;
f.origin.y = 0;
currentItemAttributes.frame = f;
return currentItemAttributes;
}
NSIndexPath* ipPrev = [NSIndexPath indexPathForItem:indexPath.item-numColumns inSection:indexPath.section];
CGRect fPrev = [self layoutAttributesForItemAtIndexPath:ipPrev].frame;
CGFloat YPointNew = fPrev.origin.y + fPrev.size.height + 10;
CGRect f = currentItemAttributes.frame;
f.origin.y = YPointNew;
currentItemAttributes.frame = f;
return currentItemAttributes;
}
EDIT !
After 3 days ,i have manage to change something that solve the problem, but makes no sense.
I have changed the content height of the collection view to be 6 times the screen size! this solved the problem. I think i am doing something wrong with setting the content side of flow and collection:
TopAlignedCollectionViewFlowLayout *layout=[[TopAlignedCollectionViewFlowLayout alloc] init];
CGRect size=CGRectMake( ([UIScreen mainScreen].bounds.size.width-collectionWidth)/2,
upperView.frame.size.height, collectionWidth, 6.5*([UIScreen mainScreen].bounds.size.height-upperLineMargin));
self.GridView=[[UICollectionView alloc] initWithFrame:size collectionViewLayout:layout];
[self.GridView registerClass:[GridCell class] forCellWithReuseIdentifier:#"Cell"];
Only now, he loads all cells at time. problem is that now i can't get to the end of the collection.