There are many obvious factors that can affect the ability to gain muscle, some are age, biological sex, genetics, consistency of nutrition, and training frequency.
We can also share some no so obvious factors that can also impact the rate of muscle growth.
Current Body Size- It is easier to gain muscle if you have a large frame/ bone structure versus a small frame.
Current Body Composition- Having a higher body fat percentage makes muscle gain harder where if your body fat is relatively lean.
Current Activity Type- Resistance training will make muscle gain easier. Inadequate resistance training and excessive cardio will make it harder to gain muscle.
Consistency- Being consistent 80% of the time or more will help you gain muscle.
Recovery- Sleeping less than 7 of hours a night will make muscle gains harder. Sleeping 7-8 hours most nights will make the gains easier.
Stress- Muscle gain is affected by excessive stress or the perception of excessive stress so keeping your stress levels managed or having the perception of appropriate stress will help you gain muscle.
So when you are analyzing your progress use these additional factors as a gauge to see what you need to adjust in your lifestyle to help your progress.