With both dogs, it does seem like it might be diet related?
One way to increase absorption of calcium is to increase D - wonder if you northern folks have a greater need for it?
But....if you are adding meat, and for some reason there is not enough calcium to balance that, that would definitely cause the low calcium. You could try to add a crushed egg shell. I used to use a combination of NOW calcium carbonate powder, and also the supplement Nutramin, for Max's calcium needs. Being poultry sensitive, he even reacted to the eggs!
Not sure about the electrolytes?