Currently aggregate checks work only with identical items that match the item key, exactly.
This behavior does not work with LLD items, since they can vary in ways you cannot sum statically.
We could do something like the the LLD Regexp filter, or use some kind of wildcard in it.
The practical scenario that i'm trying to work is to monitor all uplinks from my core routers, and create an grpsum of all interfaces. This is not possible right now because the interfaces almost always different from router to router, and i cannot match more than one interface per item.