always when you deco have a mount ... horse, beetle, ethy, sumpin.... and after placement dismount and mount and that will redraw everything and you'll see where it really is, then raise or lower as necessary
some plants are groundhuggers and just fall the lowest point no matter what you're placing them on, and that's what you'll have the most trouble with (water plants come to mind) and sometimes, for some reason there are spots in a structure that simply swallow things, tho I've found that 'feature' mostly in legacy houses