You could double check to see if Attenuator is on for any of the channels. Also if all the antenna connections are snug.
Lastly, and this is exceedingly remote, but there could be something near the scanner that is desensitizing it. A lot of electrical "things" generate a little bit of radio signal that can interfere with a scanner if they are too close to each other.
If the problem persists and all the above are checked and okay, an outside antenna (or attic) might be helpful.
KSP's current system does not use any kind of simulcast (multiple transmitters on the same frequency) so an SDS scanner isn't warranted.
Programming all of a post's channels is sound advice. Your closest tower could have a problem, so if the next closest isn't programmed, you will be out of reception until it is repaired.
I will often hear a dispatcher tell a trooper "You are going digital" (garbled), so even "they" have this issue. A unit could be in a bad area, such as on the backside of a hill, and not get to a receiver very well. KSP has fussed for years about bad coverage, so they are looking to make changes (a ways off I'm sure). Bear in mind that this system has some age on it. It started out as an analog UHF system back in the early 80s and switched over to digital perhaps just after 2000. Still, the towers, microwave, etc. have age on them.
Also just the way they go about things has opportunities for less than stellar signal. Portable radio to vehicular repeater/extender, then mobile radio to a tower. Their portables are fairly low power, so a trooper has to be close to his/her vehicle. And then if there is something where multiple units are present, having more than 1 mobile repeater in use happens and that messes up the signal.