The Harris radio (as well as all the commercial radios) have a much better receiver than our scanners unfortunately, so it's not unexpected that it would work fine when our scanners are on the fritz. I think that's the issue - you're in a sub-optimal spot for this system and you're Harris radio is able to pull it out, but our scanners are right on the fringe so decoding is hit or miss depending on the conditions. :-/
The best I think we can do is play with the antennas, and antenna location, and hope to find a sweet spot. But I'm guessing you've tried most of the obvious solutions at this point.