Why the Dragon Power Gauge Decreases in Dragon Missions

In a single Dragon Mission, even though I filled it with the exact same dragons, when I participate, the gauge value keeps decreasing. When I first did this mission, I definitely filled the gauge to 4, but after retrying a few times, I couldn't reach level 4 anymore. I was wondering why the gauge kept decreasing whenever I did Dragon Missions... The reason is.. It seems the gauge fills up less each time I get a scroll. When you get a scroll, the required Dragon Power seems to increase. Looks like I'll have to diligently level up my dragons lol