I think your problem is due to the swings of dry weather and then heavy rains...I've copied here an explanation.
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Often the problem has more to do with the moisture levels in the garden to regulate the delivery of nutrients than the amount of calcium available in the soil, and tomato rot will be more noticeable after periods of uneven precipitation such as when drought conditions are followed by periods of heavy rain.
Organic Control and Prevention of Blossom End Rot
So a better way to combat blossom end rot is to ensure that your growing beds contain plenty of organic matter to help maintain even moisture levels and by watering your tomatoes as needed during periods of low precipitation.
Some gardeners claim that planting tomatoes out in the garden before the soil has thoroughly warmed up can promote the occurrence of blossom end rot. Don’t plant those heirloom tomatoes, peppers, squash, eggplants, and watermelons out into the garden until the soil has had a chance to fully warm up.
Other precautions include avoiding cultivating too closely to the plants which may encourage blossom end rot by destroying the tiny feeder roots that grow close to the soil surface and supply moisture and nutrients to the plants.
Mulching the soil after temperatures rise will help to conserve the amount of moisture that is retained in the soil and prevent or lessen the amount of blossom end rot on your tomatoes and other vegetables."
For the past 3 years I've had early & late blight issues even though I planted in a different place each year. Lost all my tomatoes, so this year, finally, everything looks good....Its terribly frustrating to have your whole tomato crop be destroyed, especially when you are planning on canning & freezing to last the winter...
I have been having problems with early and late blight too. I put 2 tomatoes in large containers and while the tomatoes are smaller, the plants look much healthier. I may do fewer for a couple of years in pots and see if I can eradicate it from my soil. The other major pest I have is squash vine borer. I have covered the plants, moved the plants and dug the little grub buggers out of the stem. EEWW!!! I hate the things!!