Colors will always appear a bit muted on the transfer papers, but if they're too light after transfering you may have a problem. You might try printing at higher quality (thus, more ink) if you're not already - say, 720dpi instead of 360dpi.
I've also heard that while MiraCool has an incredibly soft hand, the colors aren't as vibrant as you'll get with the Transjet II/Magic Jet. Might give that paper a try as well to compare.