I agree.......late to read the posts.
Knockout Rose's need full sun to produce the best shape. The twisting trunk seems remnant of the plant growing toward the sunlight.
I also agree that these plants are not in the correct spot.......and yes fertilizing them even with extra phosphate for blooming will only hurt them.
If this customer allows, prune them back down to two feet tall and relocate them to a suitable location. After new growth starts according to your weather conditions, then add a low nitrogen fertilizer until blooms ensue.
I see people buying these plants from all over and planting them in place of other rose's that do not fair so well. The same issue of low light and poor air circulation causes any rose to be subject to leggy stems, powder mildew, blight, rose spot, stem borer and nematodes.
For me..........the problem has been Mosaic Virus on this species.