The battery is not sufficiently charged because, for example, 1. the solar module is not optimally aligned for the time of year 2. the charging time is strongly dependent on the time of year 3. the charging time is strongly dependent on the weather 4. the solar module is shaded (completely or partially). 5. the solar module is dirty. 6. the capacity of the battery decreases at temperatures below freezing point. 7. the capacity of the battery decreases with increasing service life. 8. charge controller overloaded, e.g. due to excessive power of connected loads. Check fuse.