look im natural too... you can get big it is just harder...... i dont know what your caloric intake is but it probably needs to be higher, you probably need to drop some reps and sets, and you may not add weight
my training goes basically like this:
3 exercises per body part
all three power...no isolation crap
start off doing 5 sets of 10 for each exercise
do that for 4 weeks
the change it to 4 sets of 8 for each exercise
4 weeks later change it to 3 sets of 6
4 more weeks 2 sets of 4
and then 5 sets of 2
the big kicker is to add AT LEAST 5 lbs to your lifts every week
i dont know how much food your taking in but it needs to be at least 5000 calories......... 55% carbs, 30% protein, and 15% fat.
try this for 20 weeks..... break your workouts down to monday chest and tri's, and shoulder raises
tuesday legs
wed off
thursday back and biceps
friday shoulder presses,deadlifts, and forearms
sat and sunday off.
also spend no longer to 45 minutes to an hour in the gym everytime you train....... if you have to train more than once a day is fine, just make sure that each time you dont train longer than 1 hour.
good luck