"continue reading' tab does not display full post

Hi,
If I go to the blog tab on my homepage, it takes me to the full length post. Great! But if I try to reach the post under ‘recent posts’, ‘categories’ or ‘archives’ in the side bar, it take me to a partial post with a ‘coninue reading’ button which when pressed only gives me the same partial post.
I am a relative newbie and would really appreciate some help here…

Mel