Why can not my dog jumps on the couch, but jump?

He is a chihuahua male 6 months old. And for some strange reason, can not jump on the couch. He has a dog of a brother who can jump on and off the couch and the bed very well, but the other will only be maintained on its hind legs and act like he wants and only crystals and bark at the other dog on the sofa. I have tried to persuade him treatment and he will take little jumps trying jump over but can never jump. Now if I put him on the couch or bed that will jump well. I've also seen jumping into the air to try to catch flies, so I know he can jump! Is there anything I can do to help or should I take him to the vet?

because they keep picking him up

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