The watts/gallon rule is outdated, as it isn't just the quantity, but also quality of light (four watts/gallon of VHO is different than four watts/gallon of MH, for example).
What size is the tank, and what type of lighting are you using? I kept maxima and crocea clams under VHO on a 58g tank for quite some time, and they always looked and behaved as though they were healthy. I say "for quite some time" in that I eventually switched to MH when I started keeping some more light-demanding SPS, not that they eventually went downhill.