Answer: I would expect the roses in full sun to bloom better than the roses in part sun, but as they are all blooming the same then I would look at your watering and your soil and your pruning. They need to be actively growing in order to keep blooming, but fertilizer is not the only factor.
In spring, you need to cut them back by about a third, more if needed to remove winter damaged canes. Beginning next year you may also want to thin out some of the oldest canes to allow air and light into the center of the plant. Although not strictly necessary, a very light shearing to remove old faded blooms can encourage faster reblooming. Remove as little foliage as possible if you do this.
You would fertilize at that time with a slow release fertilizer such as 10-10-10 per the label directions. You may need to fertilize again depending on what kind you use, but do not fertilize in the fall. A top dressing of good quality compost each spring and mid summer is also a good idea as it will help keep the soil healthy and also provides micronutrients.
Using a year round organic mulch is helpful in feeding the soil slowly as it breaks down over time, and for keeping the soil moisture level steady. The mulch should be two to three inches thick. FLuff it up and replenish as needed.
You need to water to keep the soil evenly moist but not sopping wet. To know when to water, dig into the soil with your finger. If it is still damp, do not water yet. When you do water, water slowly and thoroughly so it soaks down deep to the roots. After watering, wait a few hours and then dig down and see how far it soaked in; sometimes this is surprising.
I would also suggest you run some basic soil tests to check both pH and fertility. If your soil is very acidic you may need to lime to raise the pH. The only way to know that is to test the soil. Your local county extension should be able to help you with the tests and interpreting the results with the goal of growing roses. I hope this helps!
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