1. Reason they don't show up could be because the Pixelfed instance your on, blocked the one that they're on or vice versa. Also could be a moderated single account.
2. Reason you don't see their posts is probably because nobody from your instance followed them yet. Fediverse works by follow relationships and posts are only sent to instances that "request" them via following or boosting. To remedy this, if you can see the user but not the posts, you can copy the link to the post and paste it in your instances search bar, which should pull the post to your instance so you can interact with it.
Hope this is clear enough and helps you understand.