If we assume the pic output is working correctly then the circuit is simple, the big resistor sets the current that the regulator (5 pin part) puts out and the pic is just turning this fixed current on and off really fast at a duty cycle to match the DMX level. So 128 is half the time it is on and half it is off. This makes the led get it's rated current but only half the time. So in effect it gets an average of half the rated current and is half bright.
So likely issue is
1- the resistor is the wrong value so it has the current set too low.
2 - bad solder joint is cause the circuit to see it as the wrong value. or is limiting current to the circuit causing the same symptom.
3 - a bad regulator not regulating correctly.
4 - a bad led that is not putting out full light output for the current it is being given.
so check all solder joints and touch them up with plenty of heat and add a little solder for the flux or if you have some flux just add it to the joint before heating.
What is the voltage on the red leds + and Neg terminals at full brightness. This gives us a idea of if the led is getting what it should.
RJ