Genetics places an important roll in building muscle, but it is not the only thing. Often training methods are flawed and people try to train like the pros when they are only beginners or amateurs at best with horrible diets and in turn they blame genetics. It just really depends on a variety of factors when it comes to arms.
The most important factor I see overlooked in arm size is weight gain. People are forgetting you must gain weight to add inches to your arms. The 2nd most overlooked factor is progressive overload.
If someone is having trouble with triceps, I would question the following:
1- Are you gaining weight regularly? At least 1lb per week?
2- Are you progressing regularly in your bench press, parallel dips, or tricep extensions?
3- How frequently do you train your tricep muscles?
Usually, it's a good general rule of thumb that for every 15lbs you gain you add an inch to your arm circumference, provided that you're regularly adding weight to your exercises and weight has been gained steadily (majority being LBM).
This means on average, a person could expect to gain 1 inch to their arms about every 16-20 weeks. Of course, eventually you would need to cut weight and lose inches in your arms, but if you do it properly you would gain leanness.