This could be a boredom issue. As your older dog is probably slowing down a little, he might not be providing as much "entertainment" as your girl requires.
Usually with shorthairs, more mental and physical activity is the answer.
I'd also try adding some new entertaining features to your yard when you are away, a wading pool, new toys, treat balls, large meaty bones to chew, hidden food to be "discovered" during the day.
Or perhaps a couple of hours at doggy play group could help.