Everyone builds muscle at different rates. It depends a lot on your genes, somewhat on your age, and a little on how hard you train. Give two people the same exact program with the same exact weights and they can gain muscle at completely different rates.
Some people are deemed “non-responders,” meaning they can go through a strength training program and see no progress. This is a small group of people, but they exist. Others make incredible strides in short periods of time.
A study published this month in the Journal of Strength and Conditioning Research found that people who gain muscle quickly also lose it quickly. In the study, researchers found that people who gained muscle quickly lost much of their progress when they took 6 weeks off of strength training.
That means maintaining muscle can be frustrating for those to whom results come quickly. It means consistency is even more important than it is for someone who takes a while to progress. It also means that slow-growers can calm their jealousy, because they might have a hidden advantage of hanging onto the muscle they gain.
This can also explain large fluctuations in weight that some people experience when they start and stop strength training. Typically, changes in the scale are attributed to body fat, but that could actually make up very little of the fluctuations some people experience. If you find that your weight and muscle mass is fluctuating, remember that consistency is perhaps more important than any other factor in training.