
Fertilizer is generally only recommended the first year they are planted to help them become established.
They likely have too much nitrogen to bloom. Excess nitrogen will cause lots of green leaves and no flowers.
You can give the plants some phosphorous to promote flowering.
If you still have no flowering, you will want to do a soil test to correct the soil.
Here are some links with more information.
https://www.gardeningknowhow.com/ornamental/shrubs/oleander/oleander-growing-tips.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/phosphorus-plant-growth.htm