Is there a way to transfer posts etc from one account to another?
Pretty sure there isn't.
I made a Pieface account, but I can't see the comments and posts from my Lemmy account. What's up with that?
Lemmy and Piefed work a lot like email. Your posts are sent to a list of recipients, and only the receiving servers are aware of them. On Lemmy/Piefed, that list of recipients includes: the community you're posting in, the user you're replying to, any users you mention in the post, and users that follow you. Any server with a user on that list will get your post, any server that isn't featured on that list will not. Communities will then fan out posts they receive to all their subscribers, allowing them to reach a greater audience.
If you posted to an obscure community, then that explains why your post isn't visible on Piefed. If you registered on a smaller Piefed instance, that could also explain why they might not have received your post. Less users means that, even if the community is large, it's less likely for someone to be subscribed to it.
It's also possible that the Piefed instance has blocked your Lemmy instance. Or, if you just made your posts, that it's just taking a while for the two servers to communicate.