nerium oleander

I think they need a hot summer to develop properly, mine haven't flowered properly either this year. If it is a wet spring/summer next year, i will keep them in the greenhouse for a couple of extra months and dose them up with a high flower feed!

Mine are the same Milky, very green with unopened buds, as Hoya says, not enough sun and heat, I had them in Spain
and they were flowering all summer with very little feed and water, just the wrong climate for them

Hello from Greece where this bush / tree grows wild.

Yes, they do prefer hot summers or at least direct sun light. They also want ALKALINE soil, well draining - not too much water or feed. In the winter put them in a cold house, they can withstand short periods of frost once they are established. However pot grown Oleanders will always be more susebtible to the cold as the dirt in the planter might freeze. They can not tolerate frozen ground.

Don't spoil them all that much - let them work for their money and keep.