Absorption

The absorption and metabolism of calcium is controlled by many factors such as hormones and nutrients.

Stomach acid must be present to dissolve it
Magnesium is needed to keep it soluble otherwise it will be deposited and lead to various disorders (kidney stones, arteriosclerosis). Calcium works with magnesium particularly in regulating heart and muscle contraction and nerve conduction.
Vitamin D: Along with parathyroid hormone and calcitonin secreted by the thyroid, vitamin D helps maintain normal blood calcium levels.
Phosphorus
Manganese, copper and vitamin A

If some of these nutrients are not present in your diet, you can eat as much calcium as you want to but you will not get much of it into your blood or your bones.