I've found that you have good and bad cycles all the time and I've never been able to pin point it to anything in particulare LOL
We have 14 sites for the price of one and of the 14 I've found that the my exclusive ones do the best. On two of my exclusives we shoot requests for the members and they love it. They do more promotion for these sites to their friends and chat boards than we do
The strange thing is that I have one site which has half exclusive and this one does extremely well compared to the exclusive ones. It is a uniform and costume niche.