Diagnosis
The filter in your reusable pod is either too coarse, overfilled, or the pod itself isn't compatible with the needle system in the K-Express.
How to Fix It
First, never fill the reusable pod more than halfway with fine-ground coffee. Coarse grind is better. Second, make sure you're using the right filter lid—it should be a fine mesh. Third, and most important, when you place the reusable pod in the holder, do NOT press down on the handle to pierce it like you do with a regular K-cup. The reusable pod has no foil. Just close the handle gently until it latches.
If That Didn't Work
Your specific reusable pod might have holes that are too large. Try a different brand that's specifically recommended for Keurig machines. Some have much finer filters.
Heads Up
Grounds in your cup will also clog the exit needle over time, leading to the slow drip problem we talked about earlier. It's a cascade issue, so get this sorted.