It’s likely a lack of calories. You’ll need a surplus to gain any mass. Keep in mind that you will gain fat and muscle at the same time - if you’re already lean

It’s likely a lack of calories. You’ll need a surplus to gain any mass. Keep in mind that you will gain fat and muscle at the same time - if you’re already lean then you’ll gain relatively lean too. But understand you will get “fatter” too Sounds like if you workout already and aren’t seeing results you probably aren’t eating enough. But your workout might also not be ideal. For things diet related Alan Aragon is a decent resource. If you’re visual Jeff Nippard on YouTube isn’t bad. For calculating your daily intake requirements: https://tdeecalculator.net https://tdeecalculator.net isn’t terrible. As it sounds like you’re an ectomorph, I’d suggest going higher carb. Using the linked calculator, you should be given a macronutrient split and total calorie intake for bulking. Try following that for 4 weeks. I’d suggest picking up a scale that measures body fat too. They aren’t completely accurate, but better than nothing. Something like: https://www.amazon.com/gp/aw/d/B01N1UX8RW https://www.amazon.com/gp/aw/d/B01N1UX8RW You should get a couple of baseline readings from this in the mornings now. After 4 weeks of bulking get a couple more. Once you start to plateau, increase your calories by 3-500 (try to keep the same macronutrient split). Repeat this process. Decide what amount of body fat you are ok with and bulk until you reach that. Then decide what you’d like to do from there You should plan to bulk for 3-6 months at a time to notice a real difference. I’m sure you are already, but drink plenty of water. 2-4L of fluid per day (water, milk, etc). Keep in mind that your body can only handle around ~800ml per hour - so if you’re behind don’t try to “catch up” because you’ll end up dehydrating yourself. I’d post a source for this, but I’m on phone Make sure you eat enough fiber, don’t overdo added sugars, etc As for should you use protein supplements: Only if you cannot (for whatever reason) eat the required amount from food. I use protein supplementation, but I also get a lot from milk, meat, eggs, etc Protein supplements are not harmful and you shouldn’t experience side effects. Just don’t rely on them heavily (I use at most 2 scoops a day) and choose quality brands. There’s a lot more to gaining mass than I’ve posted here, but what I’ve posted isn’t a bad start. Make sure you’re working out with intensity, giving yourself adequate rest, sleeping enough, reducing stress, not training to failure all the time, and progressing steadily in your workouts. As others have stated, adding mass and eating more is taxing to the body in the long run. So staying lean and eating less might be more beneficial for you long term