When you right click a folder in the Windows Explorer and you choose „TortoiseSVN -> Show log“ you are not able to see all entries. Even when you change the date range on the upper right of the window you can not fetch all entries.
It seems like that older SVN Server History entries are deleted. But in fact this is not the case.
First check the date range on the upper right of the window.
After that try to fetch the „lost“ entries by pressing the „Show All“ button on the lower left of the window.