Dog having difficulty accepting new puppy?

Well, first of all, how long have you had this new pupupy and how long have you had the other one? The other dog is so used to being the only one that it feels resentful and maybe jealous that you have brought in this total stranger invading his or her space. What I would do is just make several attempts throughout the day to play with both of them at the same time. That way the other one will get to know the new one through you. Also, sometimes all you need to do is just give it some time and they wil eventually come around.