The DPD powder can be sensitive to moisture/ humidity during storage.
If yours is not in a desiccant jar, go to a local miracle-ear and purchase a hearing aid drying jar which includes a desiccant pillow which may be refreshed in a microwave when saturated. You can place the powder jar in the dryer or place the desiccant pillow in your test kit. This may be especially useful for off-season storage. Cost should be around $10.