When it's listening to a talkgroup does it drop out? Our is it only dropping out the control channel decode?
Try leaving the squelch all the way open.
Remove every site except for the one you are actually monitoring.
Remove every channel except for the live control channel.
See how that performs, then slowing start adding stuff back in if necessary to due to mobility or if they frequently change the control frequency.
Add a wildcard talkgroup for that system and leave it on that through the browse menu, as opposed to scanning.
Keep trying different locations. Sometimes sweat spots can be unique and small. It took my months to find the sweat spot in my apartment for the main P25 I monitor. And for another P25 System, which is transmitted less than a mile away from the other tower, I have to use a yagi antenna in a different location. Don't get frustrated. Be patient.
What antenna are you using?
What site on the system are you trying to monitor?
Are you scanning only talkgroups/channels on that system?