As an experienced breeder of these guys, what you need isn't so much live food, but food that the fish thinks is alive. So take some frozen mysis, thaw it out, then insert them in front of a powerhead so they shoot out. The fish will instinctively strike. It may take a few tries for it to recognize it, but once its trained, it'll be fine. So back to mysis, once it shoots out, it may float or sink slowly. At this point the fish will think its dead and not strike, so I use a pipet or turkey baster, suck it up and shoot it out again. Keep doing it, as long as the shrimp is whole. I've used this method till I started feeding chopped shrimp. If you have long tweezers, you can also dance the food around, or a probe with a long pin on it.
I would not recommend live brine, because if it trains to that, it may not train to mysis at all, and if brine is not enriched, the fish will slowly starve.
OH, its possible that you have 2 males, the big one has staked its territory in the feeding zone and may be preventing the other from feeding. Just a thought, separate out the feeding fish for a few days till the other trains. Then eventually, you'll have to trade one in till you get a female. Very hard to sex, but males will not tolerate other males, females do.
If I wanted to breed them again, I'd start with 4 or 6, put them all together, then a alpha male and female will be in the center, with secondary males all forced to the edge and the other females sort of in between. Capture the pair, move them out, then a secondary male and female will form up in the center (if you have more that is) till you have just a single sex left. Sell the extras back.