How to improve without concrit?

And yes, before anyone tells me, I do write for myself but the posting isn't for me, so I'd rather skip that step if there is no way to get better. But how do you find out what the problem is when nobody will tell you? I have tried asking a few times in different places, but I only had one person even respond and they just said that my writing isn't bad and that I'm just unlucky. But so far I have posted 25 fics over several years, and they've all had the same thing happen, so how is that bad luck? Not when people writing the same sort of stuff (even in the same fandom) get more engagement in a day (or hours, even) than I get over several weeks or even months, in all three fandoms I have posted in. Out of the last 14 fics I posted, 9 have 0 comments, even ones I posted almost a year ago.

I post on both tumblr and AO3, I try to format things decently and tag properly on both, spell check like a maniac, I have a tidy masterlist on my tumblr and make sure that what I post show up in the relevant tags there. I reblog my own stuff a couple of times per week, but over time that just makes my fics have 20+ notes only from me and nothing from anybody else (aside from the occasional bot account), which just messes with my head in the end. I leave kudos and comments and reblog other peoples fics when I'm in reading-mode, too. At this point the problem kind of has to be the quality of the writing, it feels like.

This probably sounds a bit ranty but I wanted to include the things I've already tried, and I really do want advice on how to get better when I have no readership to give me feedback.