When we had our Duet pair, I remember discovering the Active Wear cycle and it's use of more water than the Normal cycle.
It makes perfect sense that the Bulky Items cycle would use a lot more water. With the inept Affinity washer I have now, it doesn't fill with nearly enough water to effectively clean a king size mattress pad. I have to add a good three more gallons of water by hand to get things sloshing.
The Duet is a far superior, more versatile and capable machine than the Affinity could ever wish to be in its wildest dreams.
The solution to your problem would be to use the Bulky Items to wash only the things it was designed for, and perhaps good sized loads of towels to get them completely immersed as they tumble. With too few items and too much water, the pieces will just bump around at the bottom and never receive the cleaning action that tumbling provides.