Errors with base class

Hello, I am a fairly new AS3 programmer having recently transitioned over from AS2 and I am havin an error with my code that is not allowing me to use external actionscript files. Whenever I use one it automatically will change the base class of my movie clip to a blank space and then when I go to change the base class back it will let me, but then I go back into the properties panel to see that the base class is blank again! This is preventing me from doing a lot of things and it would be so great if someone could help because I couldn’t find any soultions on the internet. Thanks in advance!!!