It can be so frustrating waiting for the last few collectibles! After several days with no new collectibles (and believe me, I was looking very hard for them, and starting to despair of ever finding them) today I found a rare pebble (now have 10/12) and a rare butterfly (now have 11/12) within 5 minutes of each other. Still need 2 rare shells, and one rare beetle, sigh. But it's encouraging to hear of other people finding them!