It could be a lot of things. If you’re listening to a. repeater frequency like the main Sheriff freq or the Trunked system the repeater may be getting a weak signal from the mobiles (BSUPD has trouble when inside some buildings). Not a whole lot you can do about these. On simplex frequencies like VHF fire or EMS the mobile must be close to you to hear them
On your side the location of your scanner. You may find a better “hot spot” if you try different places around your house. If you don’t have one, an out side antenna should help with the simplex freqs.