The use of DTR or RTS, or both, is a question of what your flashing software does. I think in the past the Arduino software has used RTS, but switched over to DTR at some point. But it may do both to preserve backwards compatibility. If you find that one of them works, use that one. If you have a scope, you should be able to see the line go low briefly.
Otherwise all I can suggest is that you double check the COM port you're set for, and its settings. Actually I think the software handles the settings once you select the port. And make sure you've specified the right Atmega part.