Check the rootballs - it might be they need bigger pots. Alternatively, if they don't get enough sun, they don't flower very well.

Ah, I see! Lack of sunshine would keep flower buds from even forming. Lack of heat--and Oleanders need plenty--would keep the buds that form from opening. Also, since they are in pots, allowing them to dry out too much will kill the flower buds before they open. Oleanders are drought tolerant where their deep, aggressive roots can spread through the soil--in pots, not so much.