What state is Shiawassee County in?
Can anyone else in that state/county pick those talkgroups up?
Do you have another scanner that you can see if they work on?
Are you certain they haven't gone encrypted and the RR database just hasn't been updated?
Does the RR database have a weird location setting for that group of talkgroups and thus they're excluded from your scan list?
That will get us started on knowing how to proceed.
EDIT: I took a stab in the dark and found Shiawassee County Michigan and a list of talkgroups on the MPSCS trunked system. The location data for that talkgroup group looks OK (actually a little too big, but that's beside the point). However, the site "Owosso"'s range is fairly narrow in comparison. Click on this link:
http://www.radioreference.com/apps/db/?siteId=12254 - on the Google map on the right, do you live within the red circle? If not, your HP may not be picking up the transmissions off the Owosso site. On a trunking system, comms are generally only broadcast on sites where there are radios configured to listen to those talkgroups. So if you're trying to listen to Shiawassee County from another tower, and nobody with a radio on the system is using that tower AND the talkgroups you're trying to listen to, you won't hear anything.
If you ARE listening to the Owosso tower, and you still aren't hearing anything, I'd wonder if they are encrypted. The HP, nor any other scanner, will not stop on encrypted signals. If you get that far and can't come up with anything, check in the Michigan forum to see if anyone has any further ideas. Odds are good that someone else may be able to confirm whether or not they can hear those groups.