Could be that the plug is further down the line than you can reach with non-pro tools.
Possibilty (dependent on the age of your house) that it could be caused by orangeburg pipe:
Orangeburg pipe - Wikipedia, the free encyclopedia
Could be a vent:
How do you open a clogged plumbing air vent to a kitchen sink?
Good idea to call in a pro. Hope it turns out to be something simple.