Each voting page presents 300 photos with each photo from a different member.
The system is designed to give an equal opportunity by ensuring that all photos receive equal exposure.
The reason for this is to avoid a popularity contest where people bring in friends to win prizes.
1) You didn't vote for them yet.
2) There is a small pool of photos in the challenge at the moment.
3) The photos are new and haven't received any votes yet (once they receive 1-2 votes they'll move to the "back of the line").
4) You've voted so much that there are not enough new photos in the pool to show you different ones :)