Because you've painted thickly it's harder to get an angle with no reflections. I always turn off my flash when photographing artwork.
I've had a couple paintings that just refuse to be photographed!
Oddly enough one is a similar colour scheme to this. I think part of it is the colour of your/our lighting in combination with the colour of the painting. Sort of deadens it. In my studio I have flourescents and they give a completely different colour to the regular bulbs in a friends studio. Fortunately my new camera has an option to choose your lighting: tungsten, regular, daylight, flourescent and one other... before that I just had to do colour adjustments in Photoshop.