Is there something wrong with my Border Collie?

Ok so i have 3 Border Collies my youngest ones Ivy and Disney are very smart! When i teach them new tricks they learn in like less than a day. But my oldest one Boomer (hes 3 years old) isn't so smart. It took me 2 weeks to train him sit and even longer to teach him lay down. When we tell him no he completely ignores us and keeps on doing whatever it is. He also hates new people and my cousin sometimes he even snaps at people and growls. We have tried time and time again to get him to stop. He also is really bad at agility. Ivy and Disney are very good at it. I love him but it's just so annoying that he never listens. I tried using a clicker with him to teach him new tricks. The only thing he wants is the treat. Sometimes he well just jump up and take it out of my hand. When we tell Ivy and Disney no (which rarely happens) they stop. So is there something wrong with Boomer? We've asked the Vet and they said sometimes BC's can be mental. And yes when he was a puppy we took him to puppy classes. Instead of playing like the other puppies he just humped all of them and they were boys!! So is there something wrong?:(