Experts and BHG readers answer.
oleanders not blooming
It often takes oleander and other blooming plants several years to recover from severe pruning to once again reach blooming size. If the plants are growing well, you shouldn't need to spray anything on them to get them to bloom again.
How close are the oleanders growing to your septic system? Their roots are not especially invasive, but a weekly watering should provide the moisture the roots need in the area watered.