Each channel in the simulcast system must be aligned for phase, frequency, and amplitude - which is why you are seeing some channels that are OK, and others that are not.
Mis-aligned simulcast can cause signal strength degredation on a channel, but so can a number of other factors (differing power output, mis-aligned combiners, interference, etc.).
From what I have seen, simulcast alignment is a neglected aspect of the system and must be performed periodically. There are several factors that can send simulcast out of alignment, so it needs to be checked. Unfortunately, it takes a bit of skill, knowledge, and some expensive equipment to do it. Some places don't, or won't, invest the money in it. But, hey, it's only public safety, right? :roll: