Firstly, I agree to check the stock antennas. I have spent thousands on aftermarket antennas and have only found one that was only slightly better than the antenna that comes with the radio. Most were worse, and many had very high SWR readings.
There is one manufacturer who goes through great lengths to explain about possible “counterfeit” antennas. I think they do that to excuse their poor performance and maybe raise the perceived “value” of their antennas. (“Must be great if everyone is trying to copy them.”) My opinion only, of course, but I like to test everything scientifically back-to-back, and measure actual SWR numbers while holding the radio in exactly the same way. (Just the position of your hand on a radio can dramatically affect SWR readings!)
The other issue is exactly how the antennas were placed. UHF is line of sight, and this means that anything blocking the antenna can compromise the signal. This includes trees, buildings, etc., but the surprising thing is that UHF is quite good at finding holes in vegetation and penetrating buildings through openings. (The wave length itself is quite short.) But the number one absorber of RF waves in the UHF spectrum aside from just plain distance, is the human body. If you wear the radio on your belt, you can be 1/2 mile away, and the signal drops out completely just by turning so that your body is between the antenna and the transmitting radio. (Try it for yourself.)
My final point is that perhaps your expectations are too great. A mile is stretching it for any VHF or UHF radio, let alone one in real-world conditions. If you could find a way to eliminate the curvature of the earth and all the obstacles in between as an issue - as in, standing on one mountain top and talking to another mountain top - one could conceivably get many miles in range. In real world conditions, range can be as short as 1/4 mile.
Interesting trivia time. Did you know amateur radio operators on handheld radios have actually talking to the International Space Station when it was overhead. This was on the VHF frequency of 145.800MHz, and this shows how radios communicate through line-of-sight when there is literally NOTHING in between the antennas but sky and space.