You are hereUnder-watering
Problem: If your plant is drooping and you're sure you haven't overwatered it, than it might need more water. If you've been underwatering your plant, its leaves will look limp and lifeless.
Solution: While it can be good to let your plant dry out a bit after watering, you should always water the plant again before it's leaves start drooping. A good way to tell if a potted plant is ready to be watered is to pick it up and tell if it feels heavy or not. As plants use up all the water in their pot, it will start to get lighter. If you need something for comparison, you can get an extra pot and fill it with your growing medium. Now you can use this extra container for comparison with your potted plants as it represents the 'dry weight' of your growing medium. If you pick up a potted plant and its feels just slightly heavier than your dry pot, then you know it's time to water your plant. After a while you get a feel for how heavy your plants need to be and you may not even need the extra pot anymore.
Click here to see a time-lapse of an under-watered marijuana plant coming back to life.
1 picture taken every 75 minutes. Strain is Island Sweet Skunk. Used with permission.