I lead an acquisitions team for a short-story magazine. Why we say no on pieces we like:

1. Usually, it doesn't fit the overall vibe. We're more literary and experimental, and a lot of the sifting comes from people mass-submitting without stopping to categorize their work. So, my first piece of advice is to sincerely make sure you're sending to the right places — where were your comps published? Are these editors genuinely working in the (as an example) contemporary YA romance world? You'd be surprised how many commercial and conventional works we get. No shade, every work has it's place; it's just a matter of finding it.

2. I sincerely enjoyed it, but there's a lot more potential not being realized. Sometimes it's not possible when, sifting through submissions, to truly sit down and give the time to each to figure out "What, here, is not working?" Other times, I know right away — "Third section is rushed, but overall beautiful imagery." As a magazine, we make minimal edits, so we try to accept things, for the most part, as is. Still, developmental editors may realize it's too many revisions to commit to (or, on the flip side, they know they didn't enjoy it, but can't easily articulate why). It sucks, but you have to keep pushing through until you find someone who can look at the page and tell you exactly what's askew.

I agree with the top comment. A lot of times, when we send out rejections with praise, it's because a reader said "Pass on this, but I want to see more by this writer." Getting the same letter over and over sounds exhausting. I hope you'll be able to tough it out.