The milk container is dirty.
Follow the step-by-step guide:
To keep the frother efficient, clean the milk container every two days as described in the following steps
Then, extract the milk intake tube.
All components (except the thermal casing) are dishwasher safe, placing them in the top basket of the dishwasher. Make sure there are no milk residues on the lid components. If necessary, use the tube brush to scrape off any milk residues.
Use the tube brush to clean them thoroughly.