The site that had the link pointing to me was deindexed. As soon as it got indexed, the link immediately gave a positive boost, on that very same day. Check all pages for indexing.
I already mentioned what I use to check: the combo http://gindex.me/en/?ref=821 (speeding up indexing) + https://indexchecker.link/ (checking index, and if not in index, sending unindexed pages to gindex in 1 click) — super convenient.