Q.My Oleander Bush Flowers Are Turning Brown In Bud.
the bush seems to produce flower buds but they never develop. The turn brown and sometimes they have small millipede type insects in the buds. W have had them about six weeks from Thompson and Morgan. It has been suggested that this a disease and that we need to take them out of the soil and repot then in fresh compost. Does this sound like a sensible action.

Certified GKH Gardening Expert
They could, very well, be diseased. Though, if these are grown in compost by itself- This will be the issue. Compost is very strong without amending it with something that lacks nutrients.
This collection of articles will help you to know what care the shrub requires: