To maintain the optimum temperature, flow, and quality of your coffee, you are advised to descale your machine after every 400-600 capsules, or at least once a year. Nespresso offers this cleaning kit that has been designed for the Nespresso systems and its components.
Another reason could be is that you are using the large cup (Lungo) button for our Espresso blends, which will over extract coffee from the capsule. This means that the coffee inside will stop producing flavor after around 40ml. The coffee may also start to burn, making it taste bitter. Over extracting Espresso capsules will put unnecessary strain on a Nespresso machine’s pump. If this is the case, you will need to descale your machine more frequently.