I have read the old saying that one should focus on more weight and fewer reps for muscle gain and vice versa for endurance gain. I have also read about most exercises that a set should be between 8 and 12 reps and should be done with the weight that fatigues you on 12 reps. If you can do more reps using a particular weight, you should progress to a higher weight.
With that in mind, I decided to go with 5kg for my dumbbell curls because that's the weight that fatigues me after 12 reps, which is fine. The only problem is that I can only do one set of 12 and another of 5 with that weight. Is it okay to continue like this or should I reduce the weight to be able to do more sets? My aim is to gain muscle mass.
I face the same problem with other weight-related exercises (pecs fly, shoulder press, etc.) as well. I can do only one perfect set with the right weight that fatigues me in about 12 reps but unable to follow through with subsequent sets. If I reduce the weight, I can do more sets but then it allows me to do more than 12 reps in the first set. I fear it might be less efficient but not sure.