Did you try live brine, if available. My single Male Lyretail would only eat the live brine for the 1st week. Then I started mixing frozen with the live. He eventually took to just frozen.
He stop eating for a couple days about a month later. So, I started up with the live brine. And that fixed it. He now eats Prime Reef, frozen Mysids, frozen brine & Sweetwater Zooplankton.
I've had him for over a year and he's been fine by himself. I'm not sure about a Bartlett, but I would try that if you haven't already and if it's available. HTH