From what I know mushrooms won't stack, so if there's 6 in your town no new ones will grow. You'll need to find the remaining normal ones in order for them to grow somewhere else the next day. Press the up-arrow to look towards the sky, it will make trees near you invisible. This way you can check orchards and behind buildings.
That could be it -- there is a max of 5 (or 6?) mushrooms, so if there were rare ones no new ones would appear. I didn't get anything new, including furniture, the first couple days because I had mushrooms in my landscaping! I had to pick them all up to get new ones to spawn.