FulliGonzales:
so maybe the digitalWrite(CLK, LOW); isn't working and the digitalWrite(LATCH,HIGH); must be executed before the data shifting starts. But it is only a assumption by me...
No, the import of this is that the function is a latch, and not a D-type flip flop. A latch passes through all the data while "open", and retains the last data when it "closes" while a D-type flip flop locks in the data at the moment it is clocked, and only at that moment, no other. So you do not want the latch to be "open" as you are shifting data as the display would then be changing as you do so.
I begin to wonder in fact, whether the faint lights you were seeing were in fact, the data as it shifts through the register with the latch open whilst my more correct code shows nothing because the data is not being shifted in correctly before the latch is opened. Personally I would be writing my own shift routines rather than using "shiftOut" in order to understand exactly what was happening. I am not in a position to pen such routines for you just at this moment as I am limited by my little netbook!
FulliGonzales:
Nevertheless I'm going to try to use a timer interrupt to shift the data and set the OE and LATCH pin. Maybe this will help.
Interrupts are not going to improve things, just cause more problems. I have explained that OE is once set (LOW) and never changed since all it does is turn off the LEDs otherwise.