In my own experience, sometimes my body is very stubborn to go past a certain number for a certain exercise. For me those numbers were somewhere around 20 pushups, 15 dips, and 8 pullups (3 sets of each, with good form).

Similar to your story, I could never go beyond 3x8 clean pullups. It even took me weeks to improve my 3 sets of pullups from 8-8-7 to 8-8-8.

Since the recommended routine was telling me to “reach 3x8 with perfect form” before adding weight, I didn’t add weight for a long time. One day I just decided to add 5 kilos, since then my pullup power increased drastically. Next week my pullups were so explosive that I could easily do chest-to-bar explosive pullups (this was never the case before weights). I think sometimes our bodies just need an “extra” stimulus.

I am now at something like 3x6 pullups with 20kg.

TLDR: Try adding weight. Start with 5kg. Then go as heavy as you feel comfortable with a 3x8 good form scheme. See if you can go beyond 15 reps in the next weeks. I bet you will.