Don Juan roses are repeat flowering. Although we buy a rose and plant it. That doesn't mean, it's the plant's first year. Most plants are at least two years old at the time of marketing.
A few points to consider. Buying what are called, bare root roses. These are plants that have been field grown since being grafted (budded) they can be 2-3 yrs old if not more. The roots are often woody and will require soaking in water prior to planting.
Plants bought as container grown. These have been field grown and later potted up. Evidence might suggest that they have flowered. However time must be allowed for the roots, albeit perhaps good at the time of pottin, but now time is needed for the roots to bond with the potting compost.
Unfortunately so many gardeners have this notion about feeding plants. In the case of feeding, it is important to understand what the feed actually does. NPK are the guidlines. N=nitrogen. This will provide the plant with nitrogen, which in turn promotes greener an rich foliage. P=phospherous. This build strong roots. K=potash. This is the chemical for producing flowers and fruits. So the lack of potash could be a contributing factor. Siting of the plant can also be a factor. Without doubt, one of the best fertilizers is stable manure, but the choice is individual. I have three roses in my collection. Wendy Cussons. Super Star and Pride of Scotland. I have forgotten how old they are. None of them has ever yet flowered. May I suggest. You cut back your plants. Give them a really good mulch of manure and see what happens. Best wishes.