What can influence a tank's Mg reading

[JDigital]

I've been fighting to get my CA, Alk, and Mag up for that past 2 weeks.. Dosing every night any numbers just won't increase to the numbers I want... I don't have a packed SPS tank, so I don't know where all this could be going..

03/24/2009 Ca: 380-390 Alk: 6.4 Mag: 1155
03/25/2009 Ca: 370-380 Alk: 6.4 Mag: 1260
03/27/2009 Ca: 370-380 Alk : 6.4 Mag: 1110 NO3: Undectable
03/29/2009 Ca: 330-340 Alk: 7.4 Mag: 1230
04/01/2009 Ca: 380-390 Alk: 6.7 Mag: 1260