this may sound stupid (i have been asked this at work) but you must have live rock in your tank to get coralline

that said

your cal levals are very import like jo said, if nothing else works you can try going with products like purple up or coral boost but for the most part they don't work they just add cal into your tank (if you feel you must use a product i would use coral boost over purple up)

but the first step is testing and trying to get higher lights