Each collection completed allows your tribe to grow by 5 more.
I find that if you listen to the music, the collectibles usually show up at the beginning of the music sequence, and I have also noticed a "hissing" noise near the gong, and it also can alert you that a collectible is near (or even mushies).
Butterflies can show up everywhere, but the stones are always on the mosaic, and the beetles are always near the gong and the shells are always on the beach. I also find that if there are kids studying the strange plants near the gong, beetles will show up there more often than not.
Don't worry, it will happen. Some things just take time.
Good luck in your search!!
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May the eyes of your heart be open to the love that you are!!