KH 170, Ca 260, Can't get it up.

[Ruth]

IMO you are taking a chance dosing magnesium without testing first to see where it is at and if that is your problem. I would get a mag. test, make sure your calcium and alk/KH tests are not expired and go from there.
If you can't get test kits right away an interim measure may be to do a couple of large water changes as "most" salt mixes are pretty close to being balanced - give or take a little.