The PSC01 is set to Input B Dry Contact Closure and Output Mode 3?
The PUM01 is set to Relay Only or Relay/Sounder?
Cat5 is connecting the PUM01 Output to the PSC01 Input? Are you using one twisted pair for the connections or more then one wire for each terminal?
If you have the sounder enabled on the PUM01. Does it go On and Off when sent the X10 commands?
Disconnect the Cat5 from the PSC01 end. If you connect and disconnect a piece of wire across its Input Terminals of the PSC01. Does the WS13A function as expected? If it does then there maybe some noise on the Cat5 wire causing unexpected results.
If it does not. It maybe possible the Fluorescent light is making so much power line noise the PSC01's power line commands are not turning it Off but can turn it On when there is no noise from it. Is there any way you can temporarily wire a incandescent bulb to it and see if it then works as expected? Since the Off button on the PSC01 also does not turn the WS13A Off. I would be leaning towards the lights ballasts if electronic. Making noise or having a noise capacitor on its AC Input killing the PSC01's power line commands.
WS13 is the X10 Version of the switch. The XPS3 is the X10Pro version of the switch.
http://kbase.x10.com/wiki/WS13A