You don't come up on a Google search without really narrowed search terms. Social media isn't useful if it's hard to search.
When you are located, your services aren't obvious, and lack of pricing plus focus on "brain training" makes you appear expensive.
I don't think most responsible dog owners want "brain training" for their dogs, any more than most parents would want that for their kids when very small. They want them happy and healthy. I think if you offer a dog walking service then the emphasis should be on how bored dogs can be cooped up all day, so you work hard to tailor your walks to the dog so they are stimulated, tired out and happy when returned home. If you want a successful business then sell the dog's pleasure and that their need for stimulation and exercise is fully met while the owner works.
I think the problem is that you're selling what you want to offer, instead of what someone wants to buy. You need to have a think about what people actually want in a dog walker. Your qualifications would definitely interest me, but as an extra reassurance that the walker knows what s/he is doing. To be absolutely blunt I would never leave my dog with someone whose marketing focused on "brain training" because I can do agility etc for that myself. Focus on the fun and stimulation for the dog, and the need they have for company and exploration every day, and that when you get in from work you're exhausted and don't have time or energy for long country walks.
Have you contacted grooming parlours and vets and offered a discount code with their flyers? If you personalise them you can work out which are working for you, too.
I think your marketing/branding is a problem, and lack of transparency on costs is a problem. I don't think the USP is what you think it is - to me, it would be the fact you are clearly knowledgeable about dogs so could be trusted to care for them well, and not just doing it as easy moolah. The primary aim, though, would be that the dog is happy and stimulated, and exercised. I would actively avoid someone who trained them without my being present, I'm afraid.