All the photos in a challenge get exposure on the voting page.
But, since other members judge the photos, it's possible that some of them won't get votes.
A few tips to help get votes:
1) Swapping out a photo(s) that aren't getting votes will help.
2) Make sure your photo(s) match the challenge theme (see challenge details).
3) Check out the leading members and photos in the challenge to see which ones are getting the most votes.
*Note: The amount of exposure changes depending on your "Exposure Boost".