The milk system has not been sufficiently cleaned. The milk pipework is clogged with dried milk deposits.
Clean the milk system at least once a week.
Also clean the milk system parts in the dishwasher from time to time.
Alternatively, soak all parts of the milk system, especially the silicone plug, overnight in warm water and a little liquid dish soap once a week.
Rinse all parts of the milk system under running water.
Make sure there are no milk residues left on the individual parts of the milk system before reinstalling all the parts of the milk system.