Jeff,

You also said that the PLM is only 20 feet from the LED Bulb, yet the PLM is having a hard time communicating with the bulb.

The issues that everyone else has identified are always good to pay attention to.  But in this case, it the two devices should be able to talk over wireless regardless of the power line interference.  The fact that they can't suggests that maybe you have some wireless interference.  Insteon works on the unregulated 915Mhz band as do many many other things.

You may want to look around your house (and your neighbor's :)) for 900Mhz items, such as:
Baby monitors (a common culprit)
Cordless phones
Weather Stations
Other wireless home automation products
......

Kevin