Two possible reasons as far as I know:
a) It's been down for a few days - but I got a busy upvote again late yesterday so that should hopefully be fixed now
(b) You need a minimum amount of follower power - here's a great analysis from @crokkon and in the comments it explains how to test that:
https://busy.org/utopian-io/@crokkon/the-busy-org-free-upvote-bot-adoption-by-users-and-performance-since-nov-2017

Hope that helps!