The "8" and "30" don't seem to be fixed numbers, but - unfortunately - "always" less than I need.
Today for another data set it's 9 and 25 and I would need 15 and 35.
By the way: everything is fine when I remove the "page" variable: all groupBy entries visible.
After adding page again, the entries are restricted.
So, there seems to be a maximum number of subplots:
N = # X Groups times # Y Groups times # pages
where Jmp decides that this is just too much and restricts the levels for the levels in view.
Hm ....