couple of questions and problem re: render passes

[grue]

hi. using max 9. starting to get into big scenes that end up eating a lot of my 2gb of ram. so, thought i'd use render passes. first, problem: i rendered out this scene in 3 pases, 1. site 2. buildings 3. foliage. attached you'll see that one of my scatter bushes is very anemic. looks like 1/2 of the objects disappeared. if i render just the foliage, it's great, nice and thick. only seems to happen when i try to merge the buildings and the foliage. site and foliage looks good too. is there something i'm not doing correctly or could this just be a bud in this particular scene?

next, questions. when i render this way, using passes and then merging them, can i build up my FG and save it, then freeze it in one of the passes and still use it for all the other passes without adding to it?

thanks.
steve