TOMAT FLOWER ROT

[modeller]

I'm growing four types of tomatoes and all are growing well with plenty of tomatoes but many of the plum tomatoes which I'm growing for the first time
are going rotten on the bottom where the old flower was. Is this a characteristic of plum tomatoes. They have all been treated the same with regards feeding and watering.
I use a moisture probe to test the moisture content around the roots and the plum ones were not overwatered but nevertheless they are rotting .
Any ideas ?

[Clanad]

which can be treated with a calcium rich spray made for the purpose...

[modeller]

Thanks for your help.