Have lots of buds, but flowers do not open. Budsdry up and die. In a large planter Live in Mexico, lots of sun. Was told to give more water as soil is dry…leaves now turning yellow. So now too much water.
Oleanders can be repotted in the Spring. They do not like to have crowded roots.
Common pests on this plant are Aphids and Scale. They can cause the buds to drop.
Treat the plant with Neem Oil.
Yellowing leaves generally indicate a watering issue, too much or too little. If the roots are crowded it may not have enough soil to retain moisture.
The yellowing leaves also can be due to the insects and a combination of both issues.
Here is a link with care information.
Caution: All parts of an Oleander are poisonous!