Most calcium products are ground up sea shells, (calcium carbonate), or just poorly absorbed cheap products that can be sold as calcium (calcium citrate). That is the first issue, the type of calcium. The second issue once you have the right calcium you have to be able to absorb it. That comes through healthy stomach acid. Most people are on Tums, Nexium, or stomach acid reducers, which is a sign absorption is not happening. The third factor comes in as Omega 3 fatty acids and Iodine, and involves your thyroid gland and liver.