The site only allows you to see a certain number (10?) of levels of nesting at once. If the 'root' comment that you're trying to view grandparents of is nested that many levels in from the multi-great-grandparent you're looking at, you won't be able to go further up the thread. Changing the 'root' to the great-grandparent will allow you to go further up the thread, but the original comment won't be on the page any more when you do.

Given how that works, it's probably non-trivial to fix this - allowing arbitrary levels of nesting to show on one page probably breaks the layout. There may be workarounds, though...