H2 tag do not working

Hi there, I’ve been using h1,h2 and h3 tags for my headers. However, when I’m at the admin posting area, I could see that the h2 tag is clear and see very well but when I go to the live site, the header seem like almost same with other font size as well while h1 and h3 tag do not get affected. Could anyone teach me how to solve this ? Thanks and have a nice day ??