One of the most common reasons for lack of growth is overtraining and lack of rest. Growth does not take place in the gym, only the stimulus for growth, so by training for hours on end you are going to limit your gains. Being enthusiastic is a good tool to have but learning how to use it wisely will lead you along the road to success.
Overtraining can be identified by numerous symptoms such as:
Frequent colds
Constant injuries that don't heal
Over tiredness/difficulties in getting up
Lack of motivation/enthusiasm
Loss of appetite
Lack of 'pump' during training
Muscle and eyelid twitches
Lack of strength and muscular gains
All of these symptoms equate to overtraining and this is when you will limit your growth. When you feel these symptoms you need to look at your training regime and say "I'm doing to much!" Identifying this will enable you to correct the situation and get back on the to the road to progression more quickly. Scale down your routines and begin training hard again.
Sleep is an essential requirement for a successful bodybuilder. Burning the candle at both ends is all well and good, but it will not lead to good results from your hard work. When you sleep is when your body will repair and regenerate the quickest as the unnecessary bodily functions shut down and the body will be able to focus on troubled areas, i.e. muscle tissue repair. This doesn't mean that you shouldn't go out and display what you have built, but sitting up watching television till 3 in the morning is not going to do you any favours. Eight hours sleep a night is not always possible but it is a good target to aim for. If you have time during the day to catch 30 minutes extra sleep then do so.