Not the sharpest tack
When frustrated with training I've often heard "well; she's a bulldog." Commonly known as a tough cookie to train; even considered as thick. Dumb? I think not. I've never met a dumb dog; no matter what breed; they are all amazing. Each and every dog is an individual and as such each learn at their own pace. Often people group breeds together as a whole as far as intelligence; not good. Just like all people learn differently so do dogs. And as far as breeds as a whole; sure there are tendencies, traits and behaviors that have been bred into them over the years. But they do not all fit the breed mold.
Bulldogs are the second most popular breed that I have worked with and I constantly hear the "not the easiest to train;" remark from their owners. I am here to tell you that some of the smartest dogs I've met are bulldogs; and they have to be the most comical. They are so smart that they convince everyone that they simply can't get it. After all they are bulldogs so just let them do as they please. Smart little devils aren't they?
What you need to do when educating your dog is to find out what makes them tick. Long thought of as one of the toughest breeds to train is the greyhound. This came around by the simple fact that they are a soft dog and tend to shutdown when harsh training methods are used. They learn quite well with positive reinforcement methods of training. They are all different; some dogs have a real drive to work, some like to do it on their own, others need coaxing and then there are the ones who convince you that they just can't "get it." Some thrive on strict rules and constant working lifestyle; others are more laid back about their learning and take it on the slow train.
Knowing what your breed or mix breed was bred to do can help; finding what they love to do can make things a lot easier. But they don't all like to do what they were meant to do. I've met Golden Retrievers who would never retrieve a ball. I've seen a video of a Greyhound dock diving; a Borzoi doing agility and a Jack Russell schutzhund dog. The sky is the limit as far as teaching your dog behaviors. Not all will succeed at all the performance activities or obedience itself; but all are intelligent, intelligent in their own way. Our job is to tap into that talent; and overlook exterior appearance and preconceived ideas. Don't judge a book by it's cover.