The CM15A has a higher signal output on the power line than the PLM or PLC. When you removed the CM15A from the setup did anything else change? Like you changed X10 devices for Insteon ones? Insteon devices as two way devices absorbe some signals on the power line if not the transmitting device.
Some folks in the X10 forums use an XTB or XTB-II to blast their X10 signals around. I do not know if the XTB line is OK with a mix of X10 and Insteon.