Did you have testing for serum magnesium? Showing low by this means, is serious, and typical of people on diuretic therapy.

A serving of KAL magnesium glycinate is given here:

http://www.nutraceutical.com/search/..._index=4310878

This is old style labeling, and NOT giving elemental content.
(meaning the amount of magnesium only)

This is how Doctor's Best labels its products:
http://www.pureformulas.com/high-abs....UXm3SEkx.dpbs

Serving size 2 = 2000mg mag glycinate/lysinate=
200mg elemental

So this shows elemental is about 10% for this chelate according to Doctor's Best.

When manufacturers give spotty contents, you really don't know what you are getting.
I know Puritan's labels their malate chelate 150mg of mag from 1000mg of the total chelate. Their tablets ARE HUGE... about 1 inch long to get 150mg of magnesium.

So I am thinking your product unless each pill is HUGE... is only delivering 10% of 400mg each serving-- each pill is 200mg (or 20mg mag?) Since the word elemental is NOT used... you don't really know what you are getting in the way of magnesium.

Now that is only the % of magnesium present. Then the second factor is bioavailability.... That is how much is actually absorbed.
I cannot find reliable info on this factor for all the chelates and inorganic types out there. The NIH OIDs site used to have this on it, but it has been reworked and that is missing now.

Then the 3rd factor is acidity of the stomach. Low acid, results in poor absorption of magnesium (and calcium and other things) we know. Acid blocking drugs now have a warning on them that they reduce magnesium absorption. Since you have problems with GERD you may be in the 30% of people who develop achlorhydria (low acid) during aging.

http://en.wikipedia.org/wiki/Achlorhydria

People with reduced acidity in the stomach are not going to absorb nutrients properly.

This is why using the lotion form of magnesium, may be best for you. Morton's Epsom lotion, is the one I am using with success, and it even lowers my hypertension some so I need much less medication than I used to.
Applying this over visible veins and thinner skin (as in the inner arms and wrist areas) will give good absorption into the blood stream.

It is quite difficult to find proper labeling for oral magnesium products. Only a few manufacturers will give elemental on their labels, so consumers can figure out what they are actually getting. I had to work pretty hard to find the facts for just this one post.