One thing to keep in mind is that your rose is a young plant. Climbers generally like to get their feet under them and then put on some top growth and in the third year they are ready to bloom. So maybe all you need is a good dose of patience.
If the plant is starting to decline, I'd check the roots. If there is no room for the rose to grow the feeder roots, it doesn't matter how well you feed the rose, it can't take up the nutrients. Since it is growing, that is probably not an immediate need because those same roots take moisture up to the top growth.
Another thing you can try is moving the container to another spot. Sometimes, moving a container plant just two feet can make a difference in a plant's performance. I am not telling you this will work with your rose, but it's worth a try. You might switch it out with another large container. I don't know.
Asking "What if ... " is very much a part of the soul of a gardener.
luis_pr, you brought up a good point about checking the soil. The only nutrients container plants get are the ones we add to the container. In the US, it's almost impossible to really know the ingredients in any potting soil we purchase.
to ATP, too.