How do you get all four player to "fight"?

For the people who say they aren't getting the correct amount of monsters, is it possible ye are near the item limit? I think the monsters count as decorations since they can be moved so its possible that more can't spawn if the limit prevents them being placed. Or another possibility is that ye don't have enough blank area for them to spawn in. These are just my thoughts on this though I have no proof these are legitimate causes of the problem