Exclude/Flatten files in scenery.cfg

Hey!
I have a question on scenery.cfg file.....
I designed a douzen of heliports within a not too wide area., and I collected them all in one folder, so as to activate this complete folder in one single [area] in my scenery.cfg
It happens now that some of these heliports need an exclude line in order to remove trees and a few houses.
When I have three of such lines one after the other added to my heliport area, it seems that only one of them really gives a good result, whereas the other two exclude lines don't seem to work...
Does anyone know about rules or obstructions in restpect to adding exclude/flatten lines to the scenery.cfg?
Greetz
Jive1