It's unclear what type of connectors are on the end of the cables(s) coming from your computer.
This amp only works with mono 1/4" TRS and XLR connectors, not standard RCA connectors.
Did you try the Right one in the Left Channel to see if the Left Channel worked?
Refer to the following manual and read up on the connectors (page 7) and the use of the Stereo/Parallel/Bridge Switch (page 8). When it is NOT in Stereo Mode the unit only uses the Right Channel controls and input.
Use this switch to select the operating mode.
• STEREO mode
Channels A and B operate independently (as with a conventional
stereo amplifier). The Channel A input goes to the Channel
A output jacks, and the Channel B input goes to the
Channel B output jacks.
• PARALLEL mode
The Channel A input signal is output through both the Channel
A and Channel B output jacks. The Channel B input jacks do
not function. Channel A and B volumes can be independently
• BRIDGE mode
The Channel A input signal is output from the BRIDGE output
jacks. To adjust the volume, you must use the Channel A volume