Are you planting in pots or in the ground?? Sometimes calcium is there but the water intake is not allowing the plant to absorb it.
Some plants are just weaker, roots are not as deep, have poorer circulation. One year I had 8 cherry tomato plants in the same area and only one had blossom end rot. I change the area where I plant my tomatoes every year...and keep my egg shells to grind up to add to the bottom of the hole where I plant them as well.
I find the best fix is to decide the area where you will plant the year ahead, add some limestone in late Fall...then plant the following year.
Once your tomatoes have set, adding calcium seems to do little for the fruit that has already started to grow. Next year, add some to the bottom of the whole before you set your plants in their place as well.
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