Certified GKH Gardening Expert
Bottle Brush are very sensitive to fluctuations in temperatures.
They also will turn brown if overwatered or if water pools around the trunk.
They are very susceptible to root rot and if the disease affects the roots, they will not recover.
Make sure the water drains away from the plant.
Other reasons for the leaves turning brown are insects and fungus.
Fungicides can be applied if you suspect a fungus. Inspect the leaves for signs of disease.
Here is a link to refresh you on the care requirements.