Fruit dropping off stems.

I have been trying to figure out for a while now why every time a flower forms and starts to fruit they just drop off the stem. I know its not too hot in the basement so it can't be heat. I have a fan moving air through the system so I don't think that is it either. I can only guess there is some type of deficiency that is causing this problem. If someone has a suggestion or possible reason it is happening in my system please post a comment. This is happening on the pepper's as well as tomatoes. I have not had a single fruit develop from either. On the other side of it though, my lettuce, and cabbage have done really well.