Placing ActionScript

Basically I'm trying to make back and forwards buttons in my flash document. I've gotten to the part of writing the actionscripting and for some reason I'm having problems placing the code on the individual frames. What happens is say I go to frame one and put some script in there for an array and then I go to frame 20 to do the same thing but it won't add new information. I can't even get the little "a" symbol to show in my timeline. It seems that its not even adding the scripting to the frame but the document as a whole. I've never had this problem before so it seems really weird to me. If anyone could help me that would be great.