There’s nothing wrong with the circuit or layout — it’s been verified multiple times, both by me and many others.
The first thing I’d suggest is checking all resistor values by color band, since they can’t be reliably tested in-circuit. Here’s a chart from the Guides page if you need a reference. Download the full-color Resistor chart.
Next, reflow the solder joints, especially around everything related to the Gain control, including the potentiometer.
This only takes 5–10 minutes, and in my experience, cold solder joints or a mixed up resistor value are by far the most common issues.
One last check is to make sure there are no cracks in the glass diodes.