Vertical Scrollbar is not scrolling through all records in Folder Form

I have a simple custom folder form, When I use vertical scroll bar it only shows me first 15 records (No of items displayed on the block is 15). Dragging the scrollbar down is not helping. I have to use the down arrow key to view other records. In the below screenshot there are more than 60 records, but the scrollbar show's as if it ended with 15 records.

Any help would greatly be appreciated.

Thanks