cellForRowAtIndexPath doesn't get called after calling reloadData in UICollectionView

I have a UICollectionView which uses a subclass of UICollectionViewCell's. Initially, my dataSource contains 5 items, and when the user scrolls down, I fetch more data and add them to my dataSource, then I call reloadData.

But only 3 items are visible. and when I scroll up I can't see the rest of the items, just an empty area. I noticed that cellForRowAtIndexPath only gets called for those 3 items.

When I navigate to my parent view, and back to the view which contains my UICollectionView I can see all the items.

Note: I have implemented layout:sizeForItemAtIndexPath function as each cell has a different size.

Edit:

I partially solved the issue. I had a refreshControl and I called endRefreshing in a background thread.

Am adding images for a better demonstration of what's happening now:

• The 1st image is before fetching new data, as u can see the data is displayed perfectly.

• The 2nd image is after fetching new data, as u can see the new items take the exact height of the previous ones (the old cells) and there is an empty area, when I scroll down I can see the rest of the data, and when I scroll back up, the top cells get the right height, as shown in the 3rd image.

After I finish loading new items, I call this method

- (void)updateDataSource
{
   self.collectionViewDataSource = _manager.messages;
   [self.collectionView reloadData];
}

I checked numberOfItemsInSection method, it returns the right number of items.

and here is my layout:sizeForItemAtIndexPath

- (CGSize)collectionView:(UICollectionView *)collectionView layout:   (UICollectionViewLayout*)collectionViewLayout sizeForItemAtIndexPath:  (NSIndexPath *)indexPath 
{
// Here I am calculating the width and height of the textView which will fit the message

SPH_PARAM_List *feed_data=[[SPH_PARAM_List alloc]init];
feed_data=[self.collectionViewDataSource objectAtIndex:indexPath.row];

if ([feed_data.chat_media_type isEqualToString:kSTextByme]||[feed_data.chat_media_type isEqualToString:kSTextByOther])
{

    NSAttributedString *aString = [[NSAttributedString alloc] initWithString:feed_data.chat_message];

    UITextView *calculationView = [[UITextView alloc] init];
    [calculationView setAttributedText:aString];

    [calculationView setFont:[UIFont systemFontOfSize:14]];
    [calculationView setTextAlignment:NSTextAlignmentJustified];


    CGSize sc = [calculationView sizeThatFits:CGSizeMake(TWO_THIRDS_OF_PORTRAIT_WIDTH, CGFLOAT_MAX)];

    NSLog(@"IndexPath: %li Height: %f", (long)indexPath.row ,sc.height);

    return CGSizeMake(self.view.frame.size.width - (5 * 2), sc.height);

}

return CGSizeMake(self.view.frame.size.width - (5 * 2), 90);

}

Edit 2:

I noticed that layout:collectionViewLayoutsizeForItemAtIndexPath: gets called and it returns the right height, but cellForItemAtIndexPath still deals with an old one.