It's possible the remote machine's vim doesn't support the X clipboard/selection registers. If you type `:ver` there, does it say `+clipboard` or `+xterm_clipboard`?
If not, you may have to resort to other means to use a clipboard, such as running clipper on your Mac, plus always forwarding port 8377, plus adding necessary key bindings to your Vim config files. The Clipper page is sparse but the demo video should tell you what you need to know to use it.