Green Gage Care.
Slow fruiting greengage tree provided that it is in an open position and is has pollinators it should fruit.As you know the last two summers have not been great which might have influenced its non bearing of fruit.When it is flowering check that the other plums are from the same Pollination group. For instance Oullin’s Golden Gage is from the C Pollination group where as Victoria is from the B pollination group.Also give it a good high potash feed next spring and see what happens.