Huh! After reviewing, I think I must have mistaken something because my last two posts are not in the same community. (I also switched computers between my last comment two up and this one.)
And now, if I pick a bunch of my posts in the same community, I'm consistently seeing that those apparently don't work on Lemvotes. (Or at least I haven't found a counterexample yet.)
But the OP for this thread (to this community which is on Lemmy.world) works fine (for me, at least) on Lemvotes while my second-most-recent post (also to a community on Lemmy.world) doesn't work on Lemvotes.
And, yeah, I'm using the fediverse symbol link for all these tests. Maybe Lemmy.world blocks some communities for federation but not others? That's about the only theory that makes sense to me off the top of my head so far.