In my first few years I had a lot of sites that were flops, the plants just didn't thrive there. I picked great spots in terms of privacy but because they took forever to get there so I couldn't bring in much. What I found out the hard way was how to find the good spots with preexisting great soil.
1)Stay away from acidic soil, usually pine trees and swampy areas are an indication of that
2)Marijuana needs soil that has good drainage, loose soil with a bit of sand in it is what it prefers
3)It is a bit late now but the vegetation growing there will tell you if its a good site. I've got two great sites, I can barely walk through the thick rich overgrowth that sourrounds the place mid summer

there is a learning curve in all this, don't expect to be greatly successful your first year, you will get better and better. spread them out to different locations, never put your eggs all in one basket