WeedyBoyWonder is right...if it can be done discreetly, you should add artificial lighting to the greenhouse to supplement what the sun can no longer provide. Also, depending on where you live, it might be a good idea to put a space heater in there, too. Chilly temperatures can stunt plant growth, and may be what's causing your plant(s) to mature slowly. Adding an HID light would also help to keep temperatures up through the coldest parts of the day/nights.