It is likely an issue with light intensity. And red leaf plants need more intense light than green, because the leaves appear red due to reflecting red light. But red is also the primary photosynthesis wavelength (along with blue) so this means the plant needs more red light. Now, do not take this to mean adding red light, that is not the way to go; it means the intensity of the white light (high in red, blue and green) has to be increased. However, increasing the light may be detrimental to the fish, and other plants. I did find that the "red" leaf crypts did fairly well in my moderate light tanks. But the bright red won't.
