Several causes will produce what you're describing. I will list them off:
1. A solder joint on either side of the 22.1K resistor between T1 and T2 is cold or not connecting well.
2. A solder joint on the wire going from T1 to A6 is cold or not connecting well.
3. The solder joints and connections on A4/A5 are not conducting properly, and only half of the 12AU7 is conducting. This will be visible from the top side by only one half of the 12AU7 having glowing dots at the top and/or bottom of the plate structure.
4. The HLMP-6000 diode is damaged. This can be confirmed by measuring the DC voltage at pin A8. If it's about 12V DC instead of 1.57V DC, the LED has been overheated or broken. This can be fixed by wiring in a wire jumper from A3 to A8.
5. The ground jumper that connects the upper and lower ground of the volume pot was not installed, or A7 is not seeing a solid ground reference. This can be confirmed by measuring the DC voltage at A7, which should be 0V. Be careful not to hit A6 when measuring this terminal, and for some measure of protection, you can turn the volume pot all the way up when taking this measurement.
Let us know what you find!
-PB