yeah, my RBTA isn't too fond of mysis either. Try a different type of food like the krill, if that doesn't work, try raw shrimp or scallop or mussel. Very small piece should be smaller than the size of the mouth opening. Smaller meals more often are much better, so a little piece every day or every other day will be good. It may also be catching food from other sources. Tell us about your tank. How long running, what type (reef I assume), what are your parameters? Oh, and pictures are always a good thing. Post a pict of the nemmie and let's take a look to rule out any other issues.