Discussions

I recently adopted a 1 year old dog(lab).  
She appears to have some training. My concern is that she knows her sit and lay down but when asked to do it her ears go back and she backs away before doing it ( or does it slowly or doesn't do it at all) She appears very nervous...i don't know much of her history but would anyone know why? This only happens when I ask to sit or lay down...other commands she is fine. She also sits atomatically while fetching and when she wants to be petted, without this nervouse behavior.  
Also, I know this will take time and patience to work through but I how do I help her not react like that?  
Any ideas? Just curious why.