I sent a couple zpl files to the printer to test it and it worked fine.
After that I was receiving base64 encoded zpl data from UPS and creating a file and sending it to the printer via a Java applet, and now any zpl commands or files I send to it are doing nothing.
I put it in diagnostic dump mode, sent some commands, then hit feed and it just printed a list of the commands it was sent.. so they're going through at least, but it won't print them.
It prints other data fine, for instance when sending an image to it.
You have to print EPL code to a LP2844 printer. It does not respond to ZPL code.