The short answer
Usually one of these: sets too far from failure, no progression in load or reps, too little volume, not enough protein or food, too little sleep, or a program changed too often. Check them in that order: most people start progressing again by fixing one or two of them.
Your sets are too easy
If you could do five more reps at the end of your sets, they do not stimulate much. Aim to finish most sets with one to three reps in reserve.
You do not progress
The same weight for the same reps for months gives the muscle no new reason to grow. Write down every set and add a rep or a little weight whenever you can.
Too little or too much volume
About ten hard sets per muscle per week is a good starting point. Much less is often not enough; much more can exceed what you can recover from.
You do not eat enough
Building muscle takes protein (about 1.6 g/kg a day) and enough energy. If your weight has not moved in months and you want to gain muscle, add 200 to 300 kcal a day.
Sleep, patience and consistency
Sleep at least seven hours, keep the same program for 8 to 12 weeks and look at the trend over months. In the first year, a beginner can often gain around half a kilo to a kilo of muscle a month at best, and much less afterwards.
Put it into practice
About this guide
STARK editorial guide. Examples explain how to use the library and record training; they are not individual coaching.
Training principles: ACSM — Resistance Training Guidelines (2026).
The session examples come from the STARK program catalogue linked above.