I have 2 mature apple trees that I planted some 10-15 years ago. The 1st few years the red apple produced great but the yellow apple never produced much. Last year I planted 2 more apple trees ( another red and another yellow apple) to help with pollination. My 2 mature trees get a ton of flowers but no fruit. This year I can not blame frost due to the warm winter so I am lost for reasons that these mature tress won't produce fruit after the flower phase.