Peppers rotting on the plant

I am growing peppers in my polytunnel and the fruit of the bell pepper plants (Beauty Bell) are going soft and rotting at the bottom. I have lots of peppers on each plant and it starts to show when they are about 2 or 3 inches in size.  There is no problem with my other variety of peppers which are the long variety also there is no problem with air flow as one door sometimes both are open most of the time. Can anyone offer any advice please?