Maybe you are not feeding enough. I did an experiment a few years ago on overcrowding. I kept 3 blue yellow-tail damsels, 1 four-stripe damsel, 1 domino damsel, 12 green chromis, 1 kole tang, 1 yellow tang, 1 convict tang, 1 falco hawk, 2 chalk basslets, 1 royal gramma, 1 six-line wrasse and a host of shrimps, crabs, snails, and other invertebrates in a 20 gal for over a year. The tank was full of live rock for hiding places, but with so many animals nothing could really hide for more than a minute or so with out defending its hole. I fitered it with a 10 gallon refugium full of caulerpa.
All 12 of the chromis lived and grew from about 2 cm when I got them to 6 cm or so by the end of the year. They were fed frozen mysis until full twice a day and would eat from my fingers. All of the other fish lived also, but I lost some peppermint shrimp to the hawkfish and a hermit crab or two to the wrasse.