Sounds like it could be a simulcast issue possibly, which the Uniden SDS100 and SDS200 are better equipped to handle. Not impossible for other scanners to hear, but I've heard you miss a lot of stuff (I don't live in a simulcast area myself). As far as encryption goes, you can see on the system page
https://www.radioreference.com/apps/db/?sid=6916 under Jackson Count MO for example, look at the Mode column if its D its only Digital, if its DE its Digital Encrypted, you'll see some law enforcement is and some isn't. If it has a DE don't bother adding it to your favorites list.
Sometimes if I'm on a trunked system it will stop and I won't hear anything but it could have just missed the tail end of a broadcast and it just stayed on for the 2-3 default delay. While its scanning without picking up anything i'll see the signal bars jump around and thats probably just picking up the control channel broadcast, but won't stop unless a voice channel becomes active. If your used to hearing your FD radio don't expect the scanner to be just as good, a couple hundred dollar device thats a jack of all trades can't compete with a multi-thousdand dollar radio that has one job.
I would still recommend limiting the amount of sites added. If you added "channels" or talkgroups from the main database to your favorites list it will by default add all sites... so in your Sentinel software click the system (MARRS) and underneath you'll see the departments you've added and below those are all the sites (each site has numerous frequencies, the more sites the more frequencies, and then its scanning each one of those 37 times for the channels you selected) On the same system page I referenced above go towards the top to System Frequencies section, and on the line below on the right is a link "RR Locations" that will be a map you can reference to find out which sites are closest to where you are, and only leave those in your favorites list. Hope this helps.