When they reply to my comment thread I am unable to reply to them
AFAIK this is due to most (or all?) kbin users limiting what language you're allowed to reply in. Try setting your comment language to "English", then it should go through (Edit: I see you already got this advice from @thegiddystitcher as well).
when I log into kbin.social directly and post there my comment doesn’t show up on lemm.ee for some time.
Not much we can do about this - your comment shows up on lemm.ee as soon as kbin.social sends it over.