Computer lab voting issue

I run high school computer lab that is set up on a domain and each computer obtains its own dynamic IP. We are in a contest where we can vote on our picture. Before yesterday we could vote from every computer and the site would accept each vote. However, I reinstalled windows 7 on each computer as part of "clean up". They work perfectly. No connectivity issues. But now the voting site only recognizes about one third of our votes.. What could I have possibly done. It seems maybe a proxy or IP issue. Any suggestions