I want to build muscle bulk, and I've been going to the gym for a year, about twice a week, doing three sets of 12 for each exercise, and increasing the weight every two or so months. But I haven't increased muscle size at all. Is it better to do low reps with heavy weight or high reps with lighter weights?

If what you are doing is not getting the results you want, do something different. Yes, test out doing heavy weights with lower reps [5-6] for 10 weeks to find if it works for you. Check your protein intake against the PCF Calculator. You need to ensure your protein intake and calorie intake allow you to gain weight. Remember training is only the stimulus for muscle growth, you need the building blocks from food to repair and grow to get bigger.