Teams will only be added to the same divisions as last year if you have enabled that setting for your event. To check your event settings to make sure you have that option selected:
  1. Navigate to Fundraising > TeamRaiser
  2. Locate your event and click edit.
  3. Select Event Options.
  4. From the list of related actions select Edit Advanced Options.
  5. Now there will be some sub options under 3. Event Options. Select c. Define Returning Team Options.
  6. Question #7 on that page gives you the option to bring returning teams back into their divisions from last year. Make sure the check box in question 7 is checked. 
Now that option is checked any teams who are brought back in the future will be placed in the correct divisions. You can also add a TeamRaiser team to a division manually.