I just spent the last few hours trying to figure out my similar issue. What I discovered is that due to the boil-off variable/rate, it was causing my boil volume to be to high. The default setting isn't right for my equipment, so I kept missing my target gravity.
I suggest you figure out what boil-off value is suitable for your equipment and plug in the right value. You'll also want figure out the other metrics like mash-tun dead space etc. This all plays into what BeerSmith calculates as your starting boil volume.
Basically, I think you're using to much water, like me.
You'll need to figure out how much water you lose in a 1 hour boil. Also figure out how much dead space is in your mash-tun.
I used the equipment wizard and it was helpful,