All the photos in a challenge get exposure on the voting page.
Every challenge has thousands of photos so it is possible that some photos don't receive votes.
A few tips to help get votes:
1) Swapping out a photo(s) that aren't getting votes for others can help.
2) Make sure your images match the challenge theme (see challenge intro).
3) Check out the leading members in the challenge, see which photos are getting the most votes.
Note: The amount of exposure may change depending on your "Exposure Boost".