Changing the 3.3K (3K) to a lower value would not fix the problem with the circuit.
Or it could be that particular LED is dimmer than the others?
First, change the 1M to 10M and test, if you have not changed it already.
Next change 270K to 240K and test.
If 240K does not work correctly, try 220K. (No lower than 220K)
Before changing anything, if you have a chance,
I would like to know a voltage from the circuit for the 1st cell (2V).
With the probe NOT in the acid, the LED is On but not bright, what is the voltage on the 2N7000 Gate?
It could be that particular 2N2222 leaks more than most, or that particular 2N7000 has a higher threshold than most,
or something I have not thought of.
After measuring the Gate voltage, go ahead and make the changes.
Also, changing the LED resistor to 1.5K or 1K would be fine.
Indicator type LEDs tend to not appear much brighter with 20ma than 8ma.
3.3K will put about 3.5ma through the LED.
G-