Hi, I was having issues where the commands were not always being sent to the Insteon units. When I looked at the ISY logs and went over them with an installer in the area, he said that he though my PLM was going bad and recommended I buy two new ones for backup and to swap one out to disprove his hypothesis about the PLM.
I mistaken bought two of the USB PLM's earlier in the year and when I went to swap the serial one out, I discovered I had this issue. I just bought a Polisy, which will work with both serial and USB, so I will be able to validate if the PLM's are the issues that I am seeing.