A long thread, I know, but tells you everything! EmonTX to Rpi - Direct Serial Connection
First step is to use miniterm to check what is being received on the serial input (if anything).
$ miniterm /dev/ttyAMA0 115200
Remember to stop emonhub first.
[edit]
Did you make changes to emonhub for the serial connection?
Can you post the config and the log?