I have tried to lift reps around 8-12 because ive heard it aims for mass and not strength, i have high metabolism and id say i eat quite healthy. i am5'8, and i weight about 127 lbs, i do weight trainings alot for my upper body, biceps triceps, deltoids, etc, but it doesnt seem to add mass to my arms. What could be wrong with this ? =/

Are you eating enough? You don't grow just by working out, your body needs the nutrients to build new muscle. And just lots of protein isn't enough, you need to get excess calories too.

Do you know your basal metabolic rate, and how many calories you're getting?

For me, the workout is only 20% of the effort. 80% is diet and rest.

I second everything Nicolas just said. I'd look at how much you're eating...then eat more of it.

How many times a week do you train with weights ?

How long is each weight training session ?

How many sessions / exercises do you dedicate to training legs, back and chest ?

Do you do any cardio ?

Do you track your diet - i.e calories ?

I usually train 3-4 times, sometimes 3-2

Each weight training session is about one hour and 30 minutes

My routine :
***For most i perform 4 sets of 12 reps and for each a warm up at 60% of weight unless otherwiese
1. Close Grip Dumbbell Bench Press
2. Alternate Incline Dumbbell Curl
3. Barbell Squats ( Recently switched from Deadlifts to squats, i felt the effect from it was greater.
4. Barbell Bench Press (4 sets of 10)
5. clean and jerk (im not so sure if thats the official name of it) (4 sets of 7-8)
6. Crunches ( 2 sets of 75 then 1 set of 50)

I use to do cardio, but i stopped

Im not sure about my calorie intake, i usally tend to eat id say around 2000 a day, sometimes less, around 1700.

When people say diet is an important factor of weight training, what do they mean by that? I think of it as eating healthy like veggies, and lean meat, is that all there is to it? sometimes i would eat fatty foods, cause i think to myself im not really overweight so its no problem if i ate those foods at times.

Even eating plenty of carbs? The past 2 weeks i cut off carbs because i heard they turn into fat, is that true, what would you all suggest are good nutrients besides the obvious protein lol.?

No, do not cut out carbs out of your diet. They are very important, if you cut out carbs you will be starving your body of energy to lift efficiently and for your muscles to repair.

You need a balance in your diet. Most people will tell you to go eat 40% carbs/30% protein/30% good fats.

I advise you read the stickies in the nutrition and weight training/body building sections.

1700-2000 is way too low. You should be about 22-2300 probably.

Have you tried lifting for your lower body? Thats half of your body right there.

It isn't good enough just to have a repitition range. Timing is crucial as it determines how much time the working muscles are under tension. If you're using that range use slower timing. 4:0:2 is great for that range and will help greatly with your gains. As others have hammered into you already, diet is also important. 6 - 8 meals/day, 2 - 3 hours apart of moderate size. After a workout, have some fast acting carbs as your body needs them at this stage and absorbs them faster if you eat them immediately (about an hour) following your workout. And, of course, you will need a good dose of protein following a workout. carbs: protein: fat, 55: 20: 25.

4:0:2 what does that mean? 4 secs to lift it, keep it there for 0 secs, release in 2 secs?

Yep, that's basically it, except you've got the 4 and the 2 mixed around. It's just refers to the phases of a lift I.e. 4:0:2 - eccentric (release) : pause (keep it there) : concentric (lift).

You say that you workout a lot, how much training is a lot? Your body needs rest days to be able to let your muscle repair and grow. If you are working out more than 4 times a week I would suggest cutting back to only 3 days a week. Plus this helps your days you are working out really count- and you get max results. Also it is not all about volume. While it is important to incorporate more sets for muscle hypertrophy you do not need to do such high reps. Your intensity should be any where from 75-100% of your max.

Also keep in mind that it takes 4-6 weeks for results to show so if you don't see changes over night, its ok

Reverse: 4 second eccentric, no pause, and 2 second concentric, although I wouldn't typically suggest a slower concentric for general strengthening (you get improved neural activity with the faster concentric)...I'd much rather add to the timing in between (technically called the amortisation phase).