Do a google search about which PWPs count against you. I can't remember them all but I know the illuminated PWPs are an item that will count against you. Could be that you have too many PWPs as compared to trees and flowers. I'd try eliminating some pwps, add trees and bring back a load of flowers from the island tours if you don't have a lot of flowers.
How many different pwps do you have? I think it could cause a problem if you don't have enough different ones. Don't take my word for it, though. This is just based on what I've read in various places.