This is a common problem. First, do you save your game after you beat like 2 or 3 people? If so, the problem comes when you beat members of one group and not all of them. The 13Devils members come out after you beat two rival groups. So for example, if you beat 2 teams completely, 1 or 2 Devils come out. For every 2 teams that you defeat completely, a Devil or two will show up and challenge you. Once you defeat the Leader of the Devils, Speed King, you will get challenged by a new rival gang called the Zodiacs. These guys are even harder than the devils. Now, there is also a situation with the wanderers that are also in the mix. You have to beat a certain number of them to release other things in the game. However, certain wanderers will only accept your challenge if you meet a certain criteria. To know what you have to do to get all the wanderers to accept you challenge and where they are located,
click here.
Let me know if that helped.