I have 2 standard oleanders in pots. They had loads of buds but never flowered. Can you tell me why, please?
Oleanders have large root structures, so a large container with good drainage is necessary.
Plants can dry out easily in containers so you will need to pay close attention to watering. Daily water and even twice a day if the temperatures are above 85 degrees F.
Also check for pests and treat with Neem Oil if you see evidence.
These articles have more information on care of Oleanders.