How long do you guys walk?
Do you include any rewarding training... or fun in your walks? Like, ask her to get up onto a wall, or sit at crosswalks, or ask her for a focus or lookback while you guys are walking? Or ask her to show you doggies (or squirrels or kitties, or deer, etc) by playing the "Where's the doggie?" game. My dogs and I do that all the time on our walks for enrichment...and treats!
I like to walk my Puma pup where we can meet people.
Bonus for her. She can earn treats, show off her fun tricks, practice her manners, and get some petting from people. She loooves to get a belly rub and will plop down at people's feet and wrap her paw around their legs to say, "heeeeyyy I like you, please rub my belly more!" Anyway, walking around people (if your dog likes people) is really good socialization on your walks. I purposely drive to places some days where I know we can socialize... and get good physical exercise.
All this helps to tire out our dogs minds as well as their body.
Do you allow her to sniff and sniff and sniff while on a walk?
Some folks only allow their dog to walk and not allow enough sniffing and investigating time. This sniffing is very mentally stimulating for our dogs. Kinda like reading the "pee mail!"
On your walks do you vary it up?
I like to walk in different directions, different streets, different neighborhoods to change it up and make it more interesting to my dogs (and me)
Do you sometimes go to a park and let your dog explore? Today on the way to work, I drove to a local park and Puma pup and I walked around the park--- and she sniffed to her hearts content. She looooves that. We practiced seeing other dogs at a distance for treats. We worked on her leash walking manners (no pulling, please) and her "wait" as people passed as on the path. When we were done, she was trashed. Ran right into her own lil room at my shop and took her little 54 lb self to bed
She is still zzzzzz.
Also, when I want my dogs to be busy by themselves, I give my dogs something safe to chew on like a pigs ear or chewy strip or bully stick. Chewing relieves stress and boredom for dogs... and keeps them nice and busy.
Sounds like maybe your dog is a bit bored or not stimulated enough. My suggestion? Up your physical exercise, fun training time, and mental stimulation! Be creative, and see what works best to tire your dog out!