Welcome, Ward!

First question: Is the player set up to automatically obtain an IP address from the network, and have you set up Jemplode to search with the "Network Broadcast" feature? If those two things are mismatched, that might possibly explain why the player isn't being detected from Jemplode after a reboot. (i.e., the player reboots and gets a different DHCP address from what Jemplode is hard-coded to find.)

However that does not explain why the player doesn't seem to have any songs after you're done synching.

Wait a sec...

Just because there's nothing on the player's screen after the synch doesn't mean that the songs didn't reach the player. Here's a question... When you sent the songs to the player, did you put them in any playlists? (And by playlists, I mean actual user-created playlists, not the section of the screen that says Artists/Albums/etc.) Did you try to select those playlists from the player's main menu? Both of those things are required before the player will start playing songs. Check that first.

When you run Jemplode on the player a second time, do the songs seem to be there in the player (as far as Jemplode's screen is concerned)?

If all of the above still doesn't result in success for you, then the next steps are going to be tricky because they require something your Mac doesn't have: A serial port. The easiest thing is to just go to a friend's house who's got a PC with a serial port and do this there.

Basically we want to make sure that the databases were correctly rebuilt when you synched. First, let's get a basic diagnostic log and have you post it here.

Then we might need to do some fixes that require the database gets rebuilt by hand. The instructions for those kinds of situations are here and here. Good luck!