If you cannot build up muscle mass in your arms (biceps, triceps, forearms) despite years of regular strength training and good nutrition, how can you gain muscle mass?

You need a calorie surplus. You also need to take in the amount of the appropriate micro- and macronutrients (fat, protein, carbohydrates). And regularly set new, progressively increasing stimuli through appropriate training intensity (and then keep to the correct regeneration time). Then, over time, is built up by force.