It has nothing to do with vegetarianism to gain muscle but not gain muscle. If you want to gain muscle more effectively and don't want to give up vegetarianism, you can choose foods with high plant protein content. You can search vegetarian athletes. The following is for reference:

Bill Pearl, who won the title of "Mr. Bodybuilding" four times. He was even named "the greatest bodybuilder of the 20th century" by 1974! Just listening to the prize he won should prove that people don't need meat to grow muscles.

Billy Symonds, the winner of "Mr. Natural Bodybuilder" in 2009-he was proud of being a vegetarian since he was a child and recently became a vegetarian. He said, "vegan bodybuilding is not only feasible, but also the most ideal state!" " "

He is also a famous Chang, who won ten Olympic medals (including nine gold medals) and ten track and field champion medals. He was named "the greatest athlete in the 20th century" and "the greatest Olympic athlete in the 20th century". When talking about his vegetarian choice and whether it affected his grades, he said: "My best grade was when I was 30 years old. I just ate vegetarian food."

Mike Dancziger, an American martial arts boxer, began to eat vegan food in 2004. "When I eat a vegan diet, the 155 pound race is much easier for me. I haven't lost any muscle. I am thinner and more energetic than before. " (Source: Zhihu. The copyright belongs to the author. )