Welcome to the forum.

You need to provide more information before anyone can help.

Things like:
baud rate from PC
True or Inverted
How is the PIC setup
What format are you trying to use - SERIN, SERIN2, HSERIN

Some code to see if the command is used correctly.

Maybe all of the above is correct and the PIC understands the serial input but the problem is in another place - stuck in a loop for example.

Question for you. What is a "billposter unwinding" ?