There are a number of things to consider. Cobb touched on a few, but I'll add that cheap phone chargers and accessories also can cause large amounts of interference.
One is the strength and distance of the station you are tying to listen too. The further away, the weaker the signal is, usually about 40 miles max from the transmitter(for the average Class B signal around me).
If you are listening to a Non-Commercial station, it could be on a translator. Those are very low powered.
The weather also plays a part(I won't go into the technical aspects of it here), and can make a station 300 miles away blast in like a local, and override the local signal.
Are there stations on adjacent frequencies?
Make sure the antenna is connected properly.
Make sure the radio is grounded properly.