When you are mulching or bagging there are a few things to keep in mind. The decks can hold a large volume of grass that is beyond the capability of the blower or mulching kit to process. They work the best when the grass is cut no more than ¾” at one time. If the grass is taller, cutting twice (once on a higher setting and then again at your normal setting) would be the best solution for maintaining a good cut quality. This allows sufficient air movement around the blades of grass so they are processed properly. If you are continuing to have clumping issues, you can also slow down. Depending on the times of year, grass type, weather conditions, you may be traveling to fast for the blades to process all of the grass properly. So slowing down will help the deck system to keep up with the cutting process.