gWell its not entirely en exact science but here are a few tips and principle to follow.
1) Look at what you are currently doing and if you have been doing it for a while then change it up. Whether its your routine, sets, reps, etc. Always remember the s.a.i.d. factor (specific adaptation to imposed demands). When your body adapts you then start to engage in maintenance. If you are maintaining then you are hardly changing.
2) Secondly look at your calorie intake and the amount of cardio you are doing. If you dont want to sacrifice cardio time then you will have to add calories to a certain degree. Fueling your body for the activity can get pretty complicated but after experimenting and paying careful attention to calorie intake vs caloric expenditure you will notice your body reacting.
3) Lastly, the overload principle. In order for muscle fibers to adapt and have reason to grow you have to challenge them to grow with adequate resistance for whatever rep scheme you are doing. Most importantly you have to remember that not everyone is the same and some people may react better to the old 10-12 rep range that for so long was considered the hypertrophy range and some people experience growth with a little lower reps and some a little higher and so on and so forth.
Also, genetics. If you dont have the genetic potential then gaining will be even harder but still possible. Don’t mistake a bigger you for a big muscle massain, its good to take your body fat percentage just as much as you weigh yourself to see if you are harming yourself or bettering yourself.