Just a guess here, Rancho Cucamunga's main repeater I believe is on top of Santiago Peak, San Berdoo and Riverside's main is near Arrowhead w/ secondary on Box Springs (at one time situated next to the KMA tower) and Temecula's is somewhere towards Mt. Palomar.
While RC, SB and Riv are all dispatched from the SB office, Temecula is on the San Diego comm center. I know it's strange. When I lived out there years ago, all I could ever get was Riv / SB. Just like you'd think that Hemet and San Jac area of the inland empire would be SB's area, it's not. That belongs to Indio.
42mhz, being low band vhf is VERY line of sight. Elsinore is a realitivly deep bowl. It may not look it from the ground but you can really see it from the air. You're pretty much underneath Santiago Peak so you should get great reception from there. Arrowhead is a stretch for anything and I think Palomar may sit back too far. Double check your frequencies. You still should be hearing something from Palomar is any case, that's alot closer that Arrowhead.
I'm sure someone here can give you a better explaination than this, but hope this gets you pointed in the right direction.