Wirelessly posted (Opera/9.80 (BREW; Opera Mini/6.0.3/27.2354; U; en) Presto/2.8.119 320X240 LG VN530)
If you're getting Mesa FD, I would think that you should be getting Mesa PD.
The only thing I can think of, is FD is being carried on the Thompson Peak and/or Shaw Butte IR sites which is where you're hearing them, and PD, at least the main patrol talkgroups that are in the clear, are only being carried on the Mesa simulcast system, which, like the RWC simulcast sites, can be a trick to monitor.
You may need to invest in a directional antenna aimed at one of the simulcast sites, in order to reduce the multipath interference issues. And the best site to aim at, may not necessarily be the closest site, because another site further behind it, or a few degrees off to either side, but still in the beamwidth of your antenna, may still cause the multipath issue. It's frequently a matter of trial and error, to find the best site amongst a simulcast system to aim your antenna at.