Mr. Burns said:
If the highest temps they see are 68-70 they might just be a little slow because of that, ideally they like it around 80-85 degrees. Also, how direct is the sunlight they are getting? If the sun is still at a low angle in the sky up there, they may not be getting as much benefit as they would under direct sunlight. You might want to put a grow light over them for part of the day to make sure they are getting enough energy.
I have a small 2x3x5 standing greenhouse I keep in my sunroom, basically just some plastic tubing for a frame and a clear piece of plastic that fits over it with a zipper for a door. I keep my plants in there when it's cool outside because it holds heat pretty well. I've also been putting a light over them lately because it's been rainy and they don't get much sun and it seems to help.
I only have four plants though, it might not be feasible with 20+. I suppose you could pick out a few of your favorites and see if it makes a difference.
Hope that helps a little and good luck!