I get a lot of soft images with telephoto lenses. I put it down to:
High ISO
Slow Shutter
Poor camera holding
Not quite right with the focus
Trying "too hard" to get good results from inadequate lighting
Experience will help. My brother would look at a photo and see the technique, I would just see a photo. My lenses need light and the images I see that are good also have light, Sun light. Drab days with slow tele lenses and moving subjects don't do justice. I'll only give up on a lens when I know I'm not the weak link, or it's heavy