Calcium supplement is a bandaid... What are you using for lighting and diet? Diet should be nutrient rich greens (NO lettuces including romaine). A UVB bulb is also vet important. Twisty style UVB bulbs are horrible at providing proper UVB output too.
Are your iguanas all together? If so it is pretty common for one iguana to flourish and another not to. They are territorial creatures so the dominant of the group will basically be using the heat sources/uvb the majority of the time.. Iguanas should be housed separately.
And red iguanas are green iguanas.. there is no difference beyond their color.. They are still the same species. There are other types of iguana species but red iguanas are more or less a color phase of green iguanas.. If you got it from a pet store it probably came from central/south america on a farm that inbreeds green iguanas with a little red, and they keep inbreeding the suckers until they have a dominant red color... Basically think of Louisiana or west Virginia only worse.