hmm... strange. never had that particular problem.
if you place too many objects on the map you'll rather encounter massive lag than missing collision detection.
if these buildings made by you? if so do they have more than 128 single boxes in the geo lod? that's the limit, more than 128 will be ignored. and did you check them for convexity? (in o²: structure -> convexity -> component convex hull) that will name the selections as well. did you give the whole structure a weight in the geo lod?
sorry, have to ask this cos these are the most common mistakes with the geo lod.
are the houses you're placing all the same model? if not, try to identify if the error is due to a particular model by placing only models of one kind together.
are you sure the error is due to the buildings, not to the player model (or other vehicles) which you test the collision with?