There are server issues where like if more than a minute (iirc) goes by from you loading up a page - while you read the whole thing, plus comments too - then you try to reply, it will not go through. Or if you reply to one person and then try to reply to another, same thing. Most often, refreshing the page seems to solve it for me. So like you can write in an external editor, refresh the page, THEN submit the reply. Kbin/Lemmy lacks "polish", that is for sure - it is undeniable that Reddit's UX is better, even if everything else about that company can go to hell.
Or if you are using an app, that would be a whole other matter that I do not know about, but I hope this suggestion helped!:-)