Pluot Purple Candy fruits not developing

Pluot Purple Candy in south facing wall of Co. Antrim greenhouse since Jan 2016, lots of branch and leaf growth. Pruned back last August.  This year, Japanese Plum used for cross pollination. Lots of Japanese Plums developing but no Pluot fruit.  Scores of pluots formed but when reaching approx’ 4 to 5mm diameter, they stop swelling, are fading yellowish and beginning to drop off.  Tree size is big enough to support several dozen fully formed fruits and has roots inside and outside greenhouse.  Peach Peregrine on north side is smaller but can ripen 12 / 14 peaches. What am I doing wrong?