It does get harder finding collectibles, the bigger the population is in the village. You can also get clumps of people by the waterfalls hiding turtle shells.
I've got 2 tribes going at the moment - one science and one magic tribe, and I completed the turtle shell collection in one tribe yesterday \:D which is my first ever complete collection! My most frustrating moment was getting the chief in my science tribe to point out a rare collectible - and she obligingly pointed out a coral that I didn't have - only to discover that the one child in the village had just turned 14 so wasn't able to pick it up.
I'm always careful to check that I have at least one eligible child now...