Within the coverage area, reception problems usually are due to obstructions such as hills and valleys in the landscape.
Seasonal weather patterns, atmospheric abnormalities, and sunspots, hills and valleys as well as man-made interference will affect radio reception. For a good discussion on how weather affects radio reception please see this Web page.
To improve reception inside a building, experiment by placing your radio and its antenna (usually the power cord) near a window.
Additionally, some retailers sell powered FM antennae that might be useful to you. A general rule of thumb is ‘the higher the antenna the better’. An external FM antenna may improve reception in those areas where the signal strength is weak.