On off relay type or dimmer type X10 wall switch?
Does the replacement switch respond to X10 commands or work with the local paddle?
Incandescent type bulbs or Dimable LED?
What wattage are the bulbs?
If the switch is a later than 2007 manufacture. It would have Preset Dim.
You may want to look at the X10 wiki page.
http://kbase.x10.com/wiki/Newer_Wall_Switches_and_Preset_DimX10 diimmer type wall switches steal power through the connected load and designed for incandescent bulbs. Some dimable LED bulbs may also work.
If it was working with 2 bulbs but glows all the time with one bulb. The power it steals to operate maybe making the one bulb always glow. Try an incandescent bulb as a test. Sales page says 60 to 500 watts.