If sample is mixing poorly at one or a few positions, the mixing pads may not be making good contact with the bag and paddle behind it. Refer to TDF Service Procedure TS010.
If sample is mixing poorly in general, an addition of flow control valves on the inrush and exhaust of nitrogen at the mixer piston will help. These flow control valves are accessible from the rear of the instrument and can provide better mixing control. Refer to TDF Service Procedure TS030. Also see link to part TDF113 Mixer Flow Control Valve Assembly.