For the MSX lovers out there....
Some games require to press a keyboard key to start (i.e River Raid requires you to press 1 or 2 to start the game).
If you want to 'simulate' those keys with a gamepad, you need to remap your controller.
I was able to do so by entering Retroarch menu during the game, and change my controller type from Retropad to Retrokeyboard. Then you can remap individual buttons to keys. For example, remap the shoulders button to "1" and "2".
CAUTION: I had to exit and re-enter the Controls menu after changing the controller type, otherwise I could not remap the buttons (discovered after a couple of hours of troubleshooting...)
Hope this helps !