Arduino mega not connecting with WS2812 LEDS

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Hi,
I've already had working program for the UNO, which the LEDs were connected to pin 12, which worked fine.
I've changed to MEGA 2560 16U2 recently, (file to big for uno...) and I've changed pin 12 to 4 as described,
i can confirm that the mega receives dmx, (ch1 controls simple LEDs, which respond perfectly,)
but the WS2812 are not responding on pin 4. (tried diff pwm pins with no success.)
with uno WS2812 work perfectly, but same setup with mega wont work apparently,(except pin 12 to 4 change)
the mega is brand new, iv'e tested pin 4 with fastled, it worked, (although not responsive as ws2812.h with dmxsimple)
please let me know what is the issue, thanks!