Have not had success with Mandarin Goby in established Reef tank

My 4 year old 150 gallon reef tank has evolved to currently hold SPS,
LPS, Fish, invertebrates, clams. It has approximately 170 lbs of live
sand, and about 350lbs of live rock. The majority of the fish
population is at least 2 years old, and the LPS, and SPS have been
doing well. My nitrates are persistent but low at about 5 PPM, and
phosphates stay at about .05. My question is regarding mandarin
gobies. Despite multiple attempts over the past year, I have not been
able to keep one alive. I add one, and will see it for a couple of
days, and then I will see it no more. When the reef tank was about 1
1/2 years old I was able to add a mandarin goby which survived for
about a year, but since then no success. I recall reading from one
source that Mandarin gobies are indeed very difficult to keep, but
frequently you are told that they should do fine if they have adequate
LR in the aquarium. Have others had long-term success with Mandarins.
They are indeed unusual and beautiful fish, and I would love to be
able to keep one happy and healthy.