terrain painting problem

hi

i bought some trees in the assetstore
now i want to paint a forest but i have a problem
i select the first kind of tree an paint some tress on the terrain
next a want to paint the empty places between the first kind of trees with the next kind but unity dont do that
i think unity wants to paint the trees on the same places likte the first ones

somebody an idea how i can paint a forest out of 9 tree types

ps sorry for my english hope you undertand what i mean

many thanks

Start with a lower density, and then when you switch trees, increase the density slightly.