I have two passion fruit plants in a pot & they a fruiting for the second year but are going wrinkly

It really depends on the stage of the fruits. The fruit will go slightly wrinkly as it's ripening, however, if it turns wrinkly and fell during early fruit form, you may need to check if the plants have received enough nutrients, water and sunlight. 

Try weekly application with 'Yates Thrive Citrus & Fruit Natural Fish & Seaweed+' as a complete plant food, specially developed for boosting healthy grown and yield of juicy fruit in citrus and other fruiting plants. 

Hope this helps and happy gardening :)