Alkalinity and calcium move with each other. Things can go off kilter with one being high and the other low. The relationship between these two is managed by your mag level.
What tests are you using to test your alk
and calcium? They are both low and mag would be fine, but the fact that dosing isn't raising anything doesn't seem too possible. Along with that, I don't know any salt mix that has both levels in that area. A large water change would be able to bring them back to acceptable levels if the readings were accurate.