When your reusable K cup is making weak coffee it could be that you're using a lighter roast or not enough coffee grounds for the size of cup you're brewing.
The best ratio of coffee grounds to water is 1:15.
So for example you can use 2 tablespoons of coffee grounds for 6 oz of coffee.
Also don't pack the grounds into the reusable K-cup which can lead to weak coffee and make sure the filter is clean as a dirty or broken filter can allow the coffee grounds to get into the coffee.
Also use clean water without chlorine and other impurities for the best tasting coffee.
A clogged needle in the Keurig can also lead to weak coffee from both reusable k-cups and regular K-cups.
The reason K-cups taste different than regular coffee is because of the difference in the quality of coffee beans.
Older coffee beans and pre ground coffee are often used in K-cups and even variations in the brewing methods.
K-cups use a single serve extraction process that is not consistent with the normal brewing temperatures.
Also K-cup grounds are often pre ground and packaged for a longer shelf life, which leads to potential flavor degradation when compared to freshly ground coffee beans.
Unlike other coffee makers, the Keurig coffee machine does not brew at a high enough temperature for the proper extraction of the coffee.
The K-cup brewing method also involves puncturing the K pod and quickly passing hot water through it, which can lead to a different extraction profile compared to traditional brewing techniques like pour over or French press coffee.
Keurig machines also don't always maintain a consistent brewing temperature, which can also impact the extraction and flavor of the coffee.
There are other higher quality K-cups available, although the quality of the coffee grounds and beans can vary.