Wrinkly peppers

Hi, I’ve grown “Jericho” peppers (in greenhouse) from grafted plants for the past three years, and have had wonderful crops each time, including this year. Having already taken about 20 fruits off the 3 plants, the remainder (although a good size) have started going soft and wrinkly. They are still ripening OK, but are very soft by the time they are red. I’ve followed exactly the same feeding, watering regimes as in the past and cannot understand what might be happening. Any ideas please? Thank you.