As someone who has previously had a lot of experience killing corals, recession from the base of sps whilst retaining colour and retracted lps, doesn't immediately scream low lights to me. Do try raising them, but as previously said, do one thing at once.
You said your parameters have been stable for about a month, how long has the tissue loss been going on? It can take a couple of weeks for sps to show the effects of parameter swings. Although I might have expected the lps to pick up by now.
Do you do water changes? Two large (50% ish) water changes a week apart followed by a couple of weeks focusing stability (testing alk and salinity every day) would probably tell you if it's a water parameter / contaminant issue rather than a light / flow issue.
You said your parameters have been stable for about a month, how long has the tissue loss been going on? It can take a couple of weeks for sps to show the effects of parameter swings. Although I might have expected the lps to pick up by now.
Do you do water changes? Two large (50% ish) water changes a week apart followed by a couple of weeks focusing stability (testing alk and salinity every day) would probably tell you if it's a water parameter / contaminant issue rather than a light / flow issue.