I still do more IR control than rs232, but I'm implementing rs232 more and more into systems. For some of the components we use there are more discrete control commands available via rs232, such as specific picture color mode commands for Elite plasmas, not available as IR. I have had to alter some command strings to add a slight delay between characters in an rs232 command string to get a device to respond correctly, and I've had to add delays when using a long wire run to a device.

If your commands are indeed working, but just not every time, I would maybe add the delay time between characters. I still consider myself a novice when it comes to the rs232, so I'm hoping some of the rs232 veterans chime in on this, as I can use the info as much as you.