coral growth

[fishguyxd]

I have always wondered why when I look at other tanks and hear everyone's corals are growing like mad you know can keep up with them (xenia or others) even after 6 months they say it just started out as a frag and now it's the size of a grapefruit or bigger So what is my problem I try to keep my water within the scope ph 8.2 ,salitity 1.023 or a bit higher Calcium say 400ish and low nitrates etc. Alk or dkh is the one i have not followed closely.
heck my zoo's take forever to sprout newheads and no I do not have my fish nipping alot. I have had the lights on alot example 12hrs on ( pc light currently). I have had some growth on corals but nothing close to others.
Should I be adding something??
So can anyone throw some idea out???