It sounds like you have a good workout program overall. Your weight
training includes cardio which helps aid recovery and allows you to get the
cross training effect which many people looking to gain muscle lack.
What is definitely off in your program is the sets and reps. In order
to gain muscle, the bulk of your weight training program needs to be high
volume. How do you increase your training volume? Increase the
number of reps you perform.
Your 3 sets of 6 reps is not a good workout program for gaining muscle.
It is a strength program which will help you increase strength very well but not
cause your muscles to hypertrophy (get larger) most efficiently.
If you have not read the page about muscle
building and periodization, it is a must read. Increase the amount
of reps, use super-sets, tri-sets and decrease your rest in between sets to 0-30
seconds to have high volume weight training workouts.
If you work out 5-6 days a week you must have a diet which is high in
calories. Make sure you eat 5-6 small meals all of which contain a
significant portion of lean protein. Make sure you consume cottage cheese
or a casein protein blend and some essential fats before you go to sleep at
night. Muscle building also requires a solid post-workout nutrition
If you have not been consuming an easily digestible, fast acting post-workout
shake or bar with a 4:1 carbohydrate to protein ratio you are missing out on one
of the most important parts of getting bigger muscles.
With a correct post-workout shake your body will be able to recovery
optimally so you are ready to give 100% on your next workout.
Not to be forgotten is rest. Make sure you get good sleep and you don't
work the same muscle group 2 days in a row, i.e. Chest & Tris on Monday, then
Shoulders on Tuesday.
Have a balanced weight training program not neglecting a single body part,
eat right, and get enough rest and you should reach your goals.
Periodization cannot be overlooked if you worry about plateau so make sure you
read the muscle building article mentioned earlier.