Why do you think you are loosing them?  Milles are some of the hardier acros in my experience.  Lots of light and flow!  Of all the acros I lost, milles resisted the strongest.....some are still hanging on
 
I got a great pink mille from Betamed (Damon), one of the shaggiest I had and had excellent polyp extension from day 1.  Maybe contact him for a known winner.  If that don't work, maybe need to take a closer look at what's going on.  GL!