The gain pot should be doing 2 things as you turn it up. Think of this pedal as 2 channels - a gain side (pretty low gain still) and a clean side. As you turn up the gain, you increase the gain on the gain side, and also gradually mix out the clean side and mix in the gain side. If you are only hearing clean boost, and the volume drops as you turn the gain up, it sounds as if your distortion side is not working at all. On the schematic that would be the network between C2 and R15
Also, this pedal was originally buffered bypass, and so any copy will be buffered bypass as well. There are some that offer true bypass as either standard or an option, but these do not hold true to the original.