This is an issue with the ground or sync, somewhere along the line you have not connected either sync or ground correctly or the amp your using is not passing it.. You will not be able to use any buttons on the chassis  without having a video sync input to the chassis ..ie a picture..

You should try removing the amp from the equation and connect directly to make sure your wiring is good..

Really the amp is not necessary and is only used to compensate for a poorly adjusted chassis of which the original wells and ducksan are, due to the lower signal from the PC. Adjusting the RGB drives and bias's usually on the neck board will get you the same result as fitting the amp would do on the original chassis..