There are several things, actually, but one of the best is to improve your antenna system. P25 systems are notoriously fickle about poor signal levels, and what you describe is one possibility.
For a better analysis we need the following;
a. Where you are (county/city/state is fine)
b. What system you are trying to hear? Go up to the blue toolbar on the left, float your mouse over Database, then select Frequency Database Use the resulting maps and pulldowns to get to your system
c. Are you using an outdoor antenna? If so, which one? Fed with what coax?
These 3 items will help folks (and more importantly, folks in your neck of the woods) to make some better recommendations
HTH...Mike