You're driving the main transistor with about 1mA base current, while you should drive it with 5-10% of the collector current.Try replacing the 3k3 base resistors with 220ohm resistors.I assume the current sense resistor is ~2.2ohm.Leo..
Some LEDs (blue) are very sensitive to ESD.Soldering them in with a non-ESD safe poker could also reduce brightness significantly.Leo..
Have you measured the actual current that is flowing, for the brighter Vs dimmer leds.?
Despite my constant current setup
I have built my entire circuit on a high quality solderless breadboard