I don't think your blade stiffener is the problem. I take the darn things off at the time of purchase anyway.
The problem is the Buxus species........Boxwoods cut only once a year isn't going to cut it.
I know that this species is a slow grower yet does grow leggy in certain areas. Leaf density on the boxwoods is sparse unless there is favorable conditions for their proper growth.
I cut boxwoods real slow across the leaf surfaces as the blades will sheer the leaves and break them off. This leaves a stubby appearance unless you train the boxwood with more cutting during the season. The new topgrowth will be leggy.............what else can you say but to either trim them twice a season. In shady areas, it may be not an option as it leads to asymmetrical less dense plants.