Pokemon-go – Are small towns more likely to not have nearby pokemon at pokemon stops

Stardust is shared amongst all of your Pokemon, and upgrading a weak Pokemon can prevent you from upgrading a stronger one. However Candies are per Pokemon evolution chain, which means the rarity completely depend on the rarity of the Pokemon.

It's generally best to save your Stardust for your stronger Pokemons (or ones you really want to play with). Especially as you'll find higher CP Pokemon as you level up.

From personal experience:

• If they have an incense up, the pokemon created by the incense will only appear for them (they are marked by the same purple thing circling around a player with incense)
• Even with different levels, all pokemon should spawn for everybody (except what has been explained at point 1). Their CP will different if you have a different trainer level though.
• If they see a pokemon you don't see, try moving closer to where it spawned for them. Sometimes they are just out of your range, and the small difference in GPS placement makes them see it, but not you.

Source: Plays with GF a lot. Sometimes we don't see the same pokemon, but when I walk 10 meters towards where it was on her map I can get it.

EDIT: To help with that, compare with your friends where the App sees you. It often happens that me and my GF are right next to each other, but in the app we are detected 50-100 meters apart.

LATE EDIT: It's been years, but something could also trigger you to not see some pokemon that others see. At some speed (idk what exactly) no pokemon will appear on your map. The speed is slow enough that being on a bus might trigger that, but barely. At the next long stop (like a red light) they should appear after a few seconds.