It could be that the person you follow is on an instance that is 'Limited' by mastodon dot social.
To check - go to their profile and see if it has been 'hidden'. Usually you can click on it again and it becomes visible.
Otherwise it could be that there is a significant time lapse between when they post and when you log on and their post is way down the line on your home feed.
Maybe message one of the admins of your instance and ask them to look into it?
I am following someone on @mastodonapp.uk, but their posts are not showing up for me. This is something that happens a lot on the bird site, but I didn't think masto used algorithms to manipulate a feed.
@christineburns@AMereSolicitor@Pollik@goatsarah I believe most admins have blocked that particular instance due to their unwillingness to combat transphobia, I know that my admin (mastodon.social) has.
@AMereSolicitor@Pollik Unless you’ve accidentally muted or blocked them the most obvious explanation would be that maybe your instance has put a server level mute or block on theirs or defederated from it. Defederation would mean that any updates on that instance aren’t received and passed on by yours. Maybe @goatsarah, who runs her own server and knows the politics, could enlighten in this case.
@essjay@goatsarah@Pollik Talking of yoghurt pots and string (bean cans and string in my day), Press for Change’s email list server was, for a while, run on 2400 baud dialup from Claire McNab’s house in Bradford. That was after our first server (on a server at a company in Australia) went tits up. So, for a while our news and forum lists depended on Claire hitting dialup several times a day. Oddly I never really noticed the difference.