I live in Colorado Springs, have the SGS4 and get pretty good reception in most places but there are a few places in town, where I notice that it drops to almost nothing. As far as radios for signal strength, I think Motorola wins hands down. Too bad I had a love/hate relationship with the Motorola. I still keep my Motorola Droid Razr which happens to be only a month old, as a back up phone. (Motorola got tired of me complaining about poor battery life and sent me a new one from their factory, even though I was near the end of my contract.)
I'm thinking you may be right with the tower infrastructure having changed in your area.
If you live in Colorado Springs, have you considered going to the corporate store on Barnes and Powers and asking what they can do about a getting you an extender? I've found that they tend to be of the better customer service stores in town.