A car freshie may stop smelling because much of its fragrance has already evaporated, but your nose can also become accustomed to a scent you experience every day. Heat, vehicle size, airflow, and the fragrance profile all affect how noticeable it feels.
Try evaluating the freshie after you have been away from the car for a while. If the scent is still extremely faint, it may simply be time for a replacement. Do not add random oils, perfume, or other liquids unless the product was specifically designed to be refreshed that way, since doing so can change how the material behaves and may damage nearby surfaces.
Think about when the change happened. A gradual fade after regular use points toward normal fragrance depletion. A sudden difference on a cold morning may reflect conditions in the car rather than a freshie that became unusable overnight.
Check the freshie under ordinary conditions after time away from the vehicle, and avoid putting your face directly against it as the only test. The useful question is whether it scents the cabin at a level you enjoy. If it does not, despite normal conditions and correct use, replace it rather than repeatedly moving it into hotter locations.