There are a multitude of reasons why your plants aren't performing:
1) Ventilation
2) Light
3) Watering Schedule
4) genetics
5) Temperature
You need to provide more specifics about your setup: Give us a quick run-down of your overall setup, and what kind of watering / nutes you are using. I had a case one time where my buddies plants did not grow on schedule, because he waiting 3 weeks too long before he put them into his growing medium, and let the roots die and sit in air for much too long which stunted their growth... Little more info (especially on watering, lights, and nutes) - will produce a better diagnosis. What type of soil bed is that / how deep is it?