I have 3 Rubin blackberries that are supposed to produce large fruit . They are 3 years old and so far I have had nothing . I haven't pruned them as they are supposed to be primo cane . Each year I get a flower or two at the top of plant , but it's now , which is far to late for a berry to grow and ripen . In the winter the canes get frosted and die . I usually cut them down in spring as they are dead anyway . The canes never get more than 3 ft high . I give them a good feed of manure each autumn and again in spring . Any advice would be most welcome . Thank you .