Stubborn or difficult to train animals may not learn under group conditions. As with most things, a little research up front may save you time and money later. Selecting the highest priced trainer does not guarantee that you will get the best trainer. Ask your veterinarian and freinds for professional and personal reccomendations. If the trainer advertises behavior modification for solving problems, ask for veterinary references.
Some medical conditions appear to be behavioral problems.
If the trainer does not have a good working relationship with at least one veterinarian he may attempt to solve a physical problem with an inappropriate behavioral solution - you and your dog will pay the price for his lack of knowledge. The best dog trainer may not be the best people trainer. If the trainer does not leave you with practical skills after the training is over, the service is of little use. Shortly after the trainer goes away your dog's skill will leave too. To take advantage of any training course, you must practice in order to be proficient.