The system is supposed to block duplicate voting by both cookies and IP addresses. When someone can't vote - it must be because someone using the same IP address had already voted in this month's poll. This can happen when there is more than one computer hooked up on the same connection (like a workplace or a family home sharing the same internet connection).
As for the purple cats - that must have been my fault
I let photoshop do an automatic caliberation of the picture and it looked fine on my screen so I went ahead with it. I now reverted to the original version.