My sales on BigStock has been flat for about 6 months. They finally started to move up in February with a big jump and then another small move up last month. I am contimplating removing iStock from my list to focus more attention on those that are making more each month. I figure with the time saved, I can shot and submit almost twice as much!

That's great that your BS sales are on the rise. I can't believe how different portfolios can be. Last month I made $10 on BS and $247 on IS. Admittedly I don't have as big a portfolio on BS as on IS, but that's because when I started the difference in revenue was so drastic that I stopped contributing to BS. But the difference isn't that great. Maybe 70% of my portfolio is on BS.

I wonder what factors make one portfolio sell well on one site and other portfolios sell well on different sites.

I suspect it has a lot to do with marketing. I used to make more on iStock before the disambiguation thing. I don't even submit there until they can streamline things more, it just takes to much work. I admit that I might make more money, but losing your love for photography because of the process is no way to die.

Another thing is that each agency promotes certain keywords, they try to "get Google" for different popular areas. (I am guessing here by the way).