I love it. Mine does that, too. He also does it when I play too roughly, or when he wants to sleep rather than play. Other people have called it passive aggression... I think they are wrong! I think it is a clear and gentle communication, "I don't want to do that." So, the question is why doesn't she want to do 'that' ?
1. I agree with Amaryllis - maybe the walk isn't long enough. Can you try walking an additional 5 minutes? Or, is there somewhere (fenced?) where you can let her off leash to go run? (I take mine to a school yard after hours. As long as we clean up, and don't bother people - it is after hours - the principal doesn't mind.)
2. When you put her out - Why? Is she underfoot, or is it time for potty (but she's not ready). Do you put her in a crate...
I'll reveal a sadistic secret: although I am an ultra gentle owner .... it is not 100% of the time. I wrestle with my dog, chase him, make him go out side in the rain (I felt guilty this winter, because it was miserable... although a mild winter). And, I taught him to go outside with 100% reliability using cruel punishment... When he was a puppy, I'd say "out" when it was the last potty time before bed. He'd take that as a cue to run around the house - punishment time. I put one hand on each hip and ran with him to the backdoor. It was harmless and painless, but he hated it, because he couldn't run away... and I was 'forcing' him to do something. But, if I glance at the back door at about 9 - 10 o'clock, he gets up, stretches, and goes to the backdoor to go out. I don't suggest that you start 'brutalizing' a 5 yo GSD, but there may be a way to start the habit.... But try to understand why he resists... before you decide that you need to force her...