If it is the kitchen sink, it may not vent the stack to the roof. It likely has an AAV under the sink. They use a crappy one from the factory. Remove and replace with an Oatey or Studor mini vent from a box store. Simple screw off, screw on. Still, check the roof vents as well because the under sink AAV will allow air to be pulled IN to let the flow go down the stack, but the roof vents need to let air OUT of the tank itself to operate smoothly. you either have a vacuum lock on the way down in the stack (AAV), or vacuum lock in the tank from air OUT (Roof Vent). Good Luck!