It could also be related (as others have mentioned) to multipath* and other interference / reception factors (i.e. not a decode problem, but a reception problem). Multipath would ideally be handled using a diversity antenna scheme, but that isn't practical for a multiband receiver like a scanner. Some other ideas and explanations might be gleaned from
TV Antenna Fixes (which is written for HDTV, but would apply to public safety, as well).
* I know that the issue technically is not Multipath...or at least not traditional multipath caused by the signal from a single source taking multiple paths to the receiver. What else to call it, though? Multisource interference?